Pontiac Grand Prix won't start, horn would honk, electrical system goes crazy

Some facts:

• My dad has an aftermarket Remote Keyless Entry
• Not aware of any aftermarket theft system, does not use keyless start
• He’s used 410 watt power inverters through the cigarette lighter but has no radar detection installed or anything like that
• The manual warns about using anything above 20 amps (not sure if this applies to the inverters)
• Right before the problem started, he had the ignition on but not running while he plugged a heavy duty light into a cable meant for a GPS to see if it would work. This caused the interior light to dim. When he went to turn the car off and pull the key out, the key was seemingly locked in. So he fully started the engine, turned it off and it still wouldn’t come out. So he started it again, pushed down on the brake, moved the gear. This time the key came out. He was able to drive the car down the street and back and thought nothing of it.

Shortly afterwards, every time he attempted to start the car the horn would sound, the lights would flash and there was a clicking sound. After trying to jump the battery the electrical system went even crazier – locking/unlocking doors, opening the trunk, lights flash across the dash and the clicking sound was even louder and more erratic.

He’s tried disconnecting and reconnecting the battery and pumping the brake 3 times with the ignition on but not running in an attempt to reset the system and that causes all the warning messages on the Driver Information Center to flash the following warning messages – “AJAR” “CHARGING SYSTEM FAILURE” “SERVICE ABS SYSTEM” “SERVICE TRACTION SYSTEM” “SERVICE BRAKE SYSTEM” “SERVICE STABILITY SYSTEM” “SERVICE TIRE MONITOR SYTEM” “BATTERY SAVER ACTIVE” and the car will make wild clanking noises that appear to be coming from around the fuse box then the interior warning and dash lights will slowly die until the car seems to be completely dead.

Any help would be greatly appreciated, we are very confused.