My 2000 Ford Mustang will not start. When I try to start it there is a rapid clicking sound. I cleaned the battery with a brush, and still the same thing. Also when I go to lock my car, the horn sounds very weak, this started when the car wouldn't start.

Sounds like a dead battery. Try taking the battery cables off and clean all the connections with a wire brush.

you must disconnect the battery for 5 minutes then when you reconnect the cables press the alarm deactivate button on your remote I believe your car will start and when you see the pats light rapidly flashing the cars in theft mode and will not start

get a wire brush clean the inside of battery terminals there is a tool to do this most auto parts stores have this its called a terminal battery cleaner

Thats usally a bad battery or bad battery cable connections. If the battery is garunteed to be good and charged, then start looking for a loose terminal or heavily corroded terminals.

Your battery cable is not making connection or clean the cable or try replacing your solenoid those are the three things you should try next.

the rapid loud clicking sound is the solenoid on your starter motor. usually indicating that the starter motor is defective or the battery needs charging or replacing at this time of year older batteries tend to die due to the cold.

1 test battery, look at battery and see if it has a window on top if so see if it is green if not it most likely needs replacing, test by switching on headlights and then trying to start the car if the lights dim or go out when you turn the key this indicates the battery is bad, if the lights stay bright this indicates a problem with your starter motor.

I hope this was helpful.

Check your ground wire for battery where it connects to your frame.

Also check/clean the connections where the positive wire goes to the starter solenoid. Any connection you run into from the battery, clean them off with a wire brush or sandpaper.

Starter Solenoid can also be failing/dead, these can be tested for free at most auto part stores if you remove them from the vehicle.

the battery is most likely low or the cables to the battery have become corroded. try a jump and clean cables. symtoms of low battery/corroded cables are a rapid click/buzz sound when turning key to start

First, make sure your battery is not dead or weak. A simple Multimeter can test your car battery.

Clean your battery cable connections at the battery and starter. Also clean where the cables connect to the battery. Cleaning means to take a wire brush or sand paper to get rid of rust/corrosion.

If the above does not remedy it, I would have your starter and starter solenoid tested for free at your local auto parts store.