I mixed up connections of battery pole with ammplifier pole

Make sure battery has a full charge and battery connections all good, positive and negative. All ends, not just at the battery. Check fuses for voltage, use a test light. Some fuses, hot all the time, some go hot with key on. Let us know what you find.

keep one battery positive + pole to system red wire...this battery negative - pole to 2nd battery positive+ pole and 2nd Negative - pole to 3rd positive +pole its negative to 4th positive pole and 4th battery negative - to it makes 48 volt and 4th battery negative to system as black
+ - + - + - + -

When you change the battery, you should take off the negative
pole first and then disconnect the positive pole. wait at least 30 min and connect the positive pole first and the negative last.
Sometimes you have to drive your vehicle at least 10 miles so the car computer will acknowledge the change.

Usually modern cars have this wires in the DIN connector of radio slot, battery (yellow wire) & under key (red wire), if you need more power to powering an amplifier or other heavy electrical load you can connect a wire directly to battery positive pole, the negative can be connected directly to car chassis, be sure that the wires used for this last application are at least 4 mm of diameter.

You may want to disconnect your battery again, First Disconnect the Negative pole first then the Positive pole. Wait 20 to 30 minutes and connect the Positive pole first, Make shure that you disconnect the Negative pole first and connect the Negative pole last. This will give the vehicle's computer a chance to reset the settings. You may have to drive your vehicle for 10 miles or more to see if this advice worked. If not, go toyour local Automotive store to seek advice, specially Auto Zone.

What model Sony stereo?

Check the ground wire, a black connected to the ( - ) pole, it might have been disconnected. Clean the battery clamp and battery terminals from rust and oxidation, tighten up the TERMINAL. Remember ( + )positive pole is RED and negative ( - ) pole is black, sometimes no plastic cover!
Taaah!

Golden rule with batterys..

The Positive terminal is ALWAYS physically larger that the Negative.(ie larger battery post ) The Negative will be the EARTH connection ( bolted to car body and engine) There may be hard to see (-) or (+) on battery leads . Colours do not matter either as manufacturers vary on this !
Good Luck

It is usually marked on the casing. +(or a red marker around the pole) is positive. If you havent removed the battery connect a bulb between one of the poles a piece of bare metal (ie engine). If the bulb lights up you're on the positive. If you have already removed the battery use a multimeter and measure voltage across the poles on the battery, if the meter gives a positive reading the red test lead is on the positve. If it gives a negative reading ie -12.35V(digital meter) the red is on negative. If you have an analogue meter(a meter using a needle) it will swing to the left when red lead is on neg.

Dead car --- Red to + ... Black to -

Hot car --- Red to + ... Black to any metal car part away from the battery. Let the hot car charge the battery for a couple minutes before you try it. Try it. IMPORTANT: DISCONNECT THE HOT CAR BLACK JUMPER FIRST, then take the black off the dead car. Then disconnect the dead car red and then the other end on the hot car.