I recently bought an 03 Monte Carlo SS and the car ran fine no codes or cel. After sitting for 2 days the car would only crank and not start. I installed an amp wiring kit and aftermarket head unit and it worked fine. While the car was turning over there was an incredibly loud backfire and I shut it off for the night. I connected jumper cables next day and the alarm started going off so I disconnected the alarm speaker in engine bay. I went to crank the car and lights were flickering with key in acc position and everything went off when cranked. Radio won’t turn on, ignition 12v wire is only reading 8v. Any idea where I should start looking for my issue?

 

How long did you keep the charger connected?

 

Wow sounds like the Aftermarket unit is draining your power. The Battery should hold at 12.4 volts to start the vehicle.
You may have compromised the battery.
You might want to disconnect the aftermarket stuff, then check the battery or charge it up.
Then if you still need the aftermarket stuff triple check the wiring so it is in correctly. Then you may want to have a switch just for it to shut it down so it isn't draining the battery while running in the car.
This is just a guess of course, but where I think I would begin.

 

 

The Monte uses power just to sit there and with the extra stuff you might want to just keep a trickle charger on it.
Sometimes if you battery is older it may not come back too.
Hope you are able to solve it.

 

You stated the car would crank but wouldn't start. At that time before the backfire was it cranking over normally ? I would be concerned about that issue. I would inspect the upper intake for cracks and see if vacuum line at the fuel pressure regulator has fuel in it. Not many reasons these cars backfire.