Please Help Can Not Figure Out Electrical Problem

[Jess4tin]

I just bought a 2003 mustang gt three weeks ago and I have been chasing an electrical gremlin. the person before had spliced in a custom alarm system and I found the tail light fuse was blown. some of the things that I have noticed is when the car warms up the battery light comes on. had the battery and alternator checked and they were working perfectly. did a parasitic draw test and everything was good. then used light probe attached the clip to the positive and probe to positive wire. it lit up so I know I have a short when the brake is pressed it strobes. then when the ecu fuse is pulled it goes out and same with pcm. I can deal with the light being on but it has a horrible miss when under load at low rpm and the shifts are very delayed. also when you come to a stop light the idle bounces up and down. also if the car is put in reverse without gas peddle being pressed the car dies but this is only when the battery light is on. please help I am stumped

 

[HawkX66]

I just bought a 2003 mustang gt three weeks ago and I have been chasing an electrical gremlin. the person before had spliced in a custom alarm system and I found the tail light fuse was blown. some of the things that I have noticed is when the car warms up the battery light comes on. had the battery and alternator checked and they were working perfectly. did a parasitic draw test and everything was good. then used light probe attached the clip to the positive and probe to positive wire. it lit up so I know I have a short when the brake is pressed it strobes. then when the ecu fuse is pulled it goes out and same with pcm. I can deal with the light being on but it has a horrible miss when under load at low rpm and the shifts are very delayed. also when you come to a stop light the idle bounces up and down. also if the car is put in reverse without gas peddle being pressed the car dies but this is only when the battery light is on. please help I am stumped

Unfortunately electrical gremlins are pretty difficult to diagnose of the web. It sounds like you're going about it the right way though with the probe etc. If the alarm system is hacked in, that's where I'd be concentrating on first. I'd carefully uninstall it. Carefully being the key word.... With the tail light fuse blowing, it sure sounds like something shorting in that circuit. Sometimes the Autozone type electrical system checkers won't tell you an alternator is on the way out or has a problem like a bad diode. If you're getting 14v or so at the battery with it running, I'd look elsewhere though.
With all the symptoms you're describing, I think you better think twice about driving it until you get it sorted out. You could be causing other damage or it might leave you on the side of the road/middle of an intersection.