Charging system problems 96 GT
First off I'm new to this forum so hello to everyone! I purchased a 96 GT in December and it has a Cobra motor, computer, and transmission in it but nothing else has been changed as far as I can tell. It ran great when I got it, then a few weeks ago I noticed my lights were dimmer than usual and the alternator gauge was dipping down to between the N and O after I drove it for more than 10 miles. I figured no big deal just swap out the battery and alternator. So I installed a brand new (not remanufactured) 200 amp alternator and a brand new Optima red top battery. Fired right up and seemed to have more power than before, the gauge was up to the M like normal and everything seemed fine until I stepped on the brakes then suddenly the gauge dropped to the N and everything dimmed. So I made sure everything was off and the doors were closed, pulled the negative battery cable, attached a test light to the terminal and cable, and the light lit up like a christmas tree. So I pulled fuses until the light went out, turned out to be number 11 Radio fuse. It does have an aftermarket stereo so I disconnected everything and tried it again with no change. I then discovered the stock amp to the Mach 460 system so I disconnected it and the light went out. However when I started the car up it still has the same problem, runs and charges normally until I step on the brakes or use the turn signals or anything electrical. I'm not the best when it comes to electrical problems so I've reached the limit of my knowledge haha. Does anyone have any ideas what could cause this? Perhaps a faulty alternator? Any help would be much appreciated!