Car runs rough/stalls when brakes pressed

Okay, where on the fuse block u want me to meter?

Edit:
There doesn't seem to be any voltage drop. Just stalls out and runs real rough when electrical load is drawn. It stalls so fast once the headlights are on but I don't see a voltage drop while it's idle with lights off.

Okay, the main positive into the fuse box doesn't voltage drop at all. Stays at 14v while running and then after stall out drops to 12v. Could issues be pertaining to PCM not being properly grounded to chassis? I was checking everything out and noticed that when I pushed down against the PCM (while the harness is plugged into it) the car will click and make noise. Then when I stop pushing it against the frame of the car the clicking will stop. Could be a wire shorting out at PCM? Or the PCM ground itself?

But if I replace the ECM then I'll have to program a different key for the car to start right? I'm also thinking it could be wires shorting out not necessarily bad ECM. It's an ex cop car I bought from auction so the amount of exposed wires is unsettling and I definitely see there being a possibility of the wires having weathered and frayed at the harness. There's just so many wires I don't know where to start with it. What if I was to solder a wire to the ECM and then ground to chassis?

Sorry for all these questions but I really do appreciate all your replies after all these weeks, lol.

Hi Roy,

Hope you're doing well. So, I've had some more free time to inspect the Impala to try and figure out this issue. Upon inspecting the PCM it does look like there's been a short circuit at the PCM and harness.

I have tried to swap out the PCM with a replacement from a junkyard but of course the VIN reads different, and this engages the anti-theft system to not allow the car to start.

I've come across a couple instructional that say to leave key in ACC for 10 minutes. As soon as anti-theft light goes away turn key to off position and try to crank again. Do this three times. And that should allow the car to crank. Although the
replacement PCM would still need to be programmed at least the car would drive.

The anti-theft light goes off after ten minutes but doing this process three times doesn't appear to by bypassing anti-theft.

Is there a step I'm missing in bypassing anti-theft?

If what I'm trying to do won't work, then can I jump the signal from a secondary PCM for only the ground connection that short circuited?