Country squire stalls under electrical load

So, me and my dad rebuilt a 87 ford crown Victoria country squire. Everything was great until we started hooking back up electrical stuff. The car will start but it idles rough and smooths out as it warms up, but the kicker, it tries to stall out with any electrical load. We have replaced everything easily replaceable, i.e. alternator, battery, some fuses and relays. Also, the car runs fine on just the battery but not just the alt. Please help. Thanks guys!

Trace the ground wire from the battery down to the block. Disconnect and super clean wire and where it connects. Oil penetrating down there can cut down on the electrical flow. Wouldn't hurt to clean the wire that goes down to the starter too, but it usually isn't part of the described problem. Be sure to disconnect from battery before doing this. That is what I'd start with as it is easiest and most common.

Trace the ground wire from the battery down to the block. Disconnect and super clean wire and where it connects. Oil penetrating down there can cut down on the electrical flow. Wouldn't hurt to clean the wire that goes down to the starter too, but it usually isn't part of the described problem. Be sure to disconnect from battery before doing this. That is what I'd start with as it is easiest and most common.

We have done this with all the grounds. even added a ground that was missing on the back of the block. We have replaced altenators and even the ecm but still nothing is working. now, we do have the EGR system disconnected, could this be the problem?

I guess now I'm gonna ask what you mean by electrical load? Headlights? Fan? Radio? Something else? All or only one?

I guess now I'm gonna ask what you mean by electrical load? Headlights? Fan? Radio? Something else? All or only one?

any sort of electircal load, weather that be headlights or fan. everything except the radio causes issues.

I gave it a day to see if anyone else had any ideas. I'm now going to suggest posting your problem to a more specific Ford/Crown Vic forum. I did some checking and this one seemed like the best choice for your situation.
https://www.fordforum.com/forum/ford-crown-victoria-27/
Good luck.