Need to know

Stop Lamps
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The brake pedal position sensor provides an analog voltage signal that will increase as the brake pedal is applied. The body control module (BCM) provides a low reference signal and a 5-volt reference voltage to the brake pedal position sensor. When the variable signal reaches a voltage threshold indicating the brakes have been applied, the BCM will apply battery positive voltage to the stop lamps, center high mounted stop lamp (CHMSL), transmission control module (TCM), and engine control module (ECM). Ground for the right rear stop lamp and CHMSL is applied at G402. Ground for the left rear stop lamp is applied at G400. The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.

Do you know what DTC'S - diagnostic trouble codes are ?
DTC B3903 02: Stop Lamp Relay Circuit Short to Ground
DTC B3903 05: Stop Lamp Relay Circuit Open or Short to Voltage

DTC C0277 06: Brake Pedal Position Sensor Circuit Short to Ground or Open
DTC C0277 07 : Brake Pedal Position Sensor Circuit Voltage Above Threshold
DTC C0277 09 : Brake Pedal Position Sensor Circuit Rate of Change Above Threshold
DTC C0278 00: Brake Pedal Position Sensor Not Calibrated
DTC C0870 03 : Device Voltage Reference Output 1 Circuit Voltage Below Threshold
DTC C0870 07 : Device Voltage Reference Output 1 Circuit Voltage Above Threshold

Your best bet mite be to take it to a qulaified repair shop .

the fuel pump relay has 4 leads. one is positive to battery, one is negative, ground, one is input voltage(on/off), and one is load (to the fuel pump +). Locate a 12 volt dc power source and connect the negative to the relay ground, and the positive to the relay input voltage lead. When 12 volts is applied to the input voltage, you should hear a click of the relay opening- additionally, if you have a multi-tester, you can also check for continuity (resistance, ohmage) by connecting the multi-tester to the 2 other leads. when the input voltage lead receives the 12 volts, you should hear a click AND the multitester should detect continuity in the circuit, and lose continuity when the 12 volt is removed from the input voltage lead.

there is no polarity in ac current as ac means alternating current and that means alternating between positive to negative back to positive back to negative and so on
only dc ( direct current ) and the center pin of the plug is designated as the positive pole has polarity, the outer case being the negative
OR Are you asking the voltage of the ac adapter and the voltage of the dc output side of the adapter

Figure out what type of switching the horn uses. is it a positive voltage or a negative voltage. Is the horn earthed and a positive voltage is applied to make it activate. Then you would switch positive through the relay.
truck may use a 24 volt DC power system then horn may need +24 volt instead of 12V . If truck is 24 Volt then alarm system may need a 24Volt to 12 Volt converter device to power alarm system. You need to check what truck uses 12V or 24 V before connecting alarm to truck power.

Check battery connections. Check voltage at fuse panel. Should have same voltage as battery. If voltage is present turn on any switch, if voltage goes away with a load follow the no voltage procedure but leave the switch on while testing.
If no voltage, check for any fuses or circuit breakers in the supply wires. Still no voltage: connect voltmeter red lead to positive at battery, black voltmeter lead to negative at fuse panel. Voltage OK problem is with positive supply to panel. If no voltage connect meter red lead to positive at fuse panel, meter black lead to negative at battery. Voltage OK problem is with the negative supply to panel.
If fuse panel is supplied voltage from the ignition switch start the voltage tests at the ignition switch.
Sometimes the positive and negative supply for the fuse panel are connected at the engine. Usually the positive at the starter solenoid, if so use that source instead of the positive battery post. Voltage must be traced from the root source to the load. Both positive and negative must be isolated and verified separately. Without one, positive or negative, there will be no voltage at the load. The load being whatever needs voltage applied to function, (light, pump, etc.)
If there is a corroded connection or wire it is possible for voltage to pass through the corrosion with no load. When a load is applied more amperage is required. The corrosion is not a good enough conductor to carry the required amperage and the voltage goes away. This corrosion can be at a terminal connection, (wire to switch, battery cable to battery lug, battery lug to battery post, etc.), internally in a switch, wire, fuse or circuit breaker. It is possible for a wire to appear to be in good condition. The outer insulation can be intact while the inner conductor is corroded, sometimes turned to green powder. This is why marine grade wire is more expensive than standard wire. Marine grade wire is tinned copper, copper conductor electro coated with tin to resist corrosion.

Computers always use DC voltage. Computer speakers receive DC voltage from the computer.
DC voltage has a Positive and Negative polarity.

In the power jack of the speaker, the Center pin is the Positive connection. The outer shell is the Negative connection.

Center hole of the DC adapter plug - Positive.
Outer shell of the DC adapter plug - Negative

Voltage 5 Volts DC.

In case you need it,

http://support.dell.com/support/edocs/dta/acs5/

bad coil

If the monitor does not have a symbol (similar to the letter 'C' with a dot in the middle), the cheapest digital multimeter can be used on the 'diode' or 'continuity' setting to check the input.
If the meter reads between 600 and 900, you are measuring the forward voltage drop of a protective diode.  Any other reading will mean that the positive probe of the meter is on the positive part of the connector.     i.e. If you are placing the positive meter lead on the center pin and the negative on the shell and it reads 600-900, then the power supply needs to have the negative applied to the center pin and the positive on the shell. If the negative lead is held on the center pin and the positive on the shell and the reading is 600-900 then the center pin is the positive input and the shell the negative.
The readings shown above mean that you are measuring the forward voltage drop of a diode meant to protect the unit against improper polarization and it will probably either die or ruin the adaptor if misapplying the polarity.
You can buy such a meter at electronics or auto parts stores for under $20 US.

well I know you can buy the battery in them from a third party but Not to sure if its really worth it. I do know there is and issue with these if they sit around for a while and not used it seems like the battery dies even more and then you can not charge it. If you come across not being able to charge it, what I did was take it apart and apply a 1.5 or a AA battery battery to the leads. You will need a multimeter to determine which is negative and positive but what I did was tape some 22 guage wire to the negative and positive ends of a AA battery and apply the positive of the AA to the positive of the battery in the treo and negative to negative. You only need to hold it there for 3-5 secs and then you should be able to charge it.

I am assuming that you already know which wires go to which speaker and all you need to know is which one is negative and which one is positive. Here is one way to find out. If you can access the speakers at all or at least see the cones, it will be easy.
Take a small 1.5v battery and connect it to the wires in question, connect one end to the positive(+) and the other to the negative (-) side of the battery, don't do it for a long time, just enough to observe movement on the speaker cones. Observe the cones of the speakers, if the cones get pushed or moves out, take note of the polarity of the battery because now you know which is which. Now if the cones, move inward or it gets sucked in when you apply voltage, then it's backwards.