1986 F150 electrical issues...?

1986 F150 4x4 4.6L 4-spd Having electrical issues- started out that the aftermarket tachometer stopped working, then the (aftermarket) radio, killed a battery (won't even take a charge). Replaced alternator aftter those issues. Truck ran for a few days. Now the truck won't start at all-no lights, radio, or ignition, and it killed YET ANOTHER battery. I have more experience working with the mechanical side of things... always avoided electrical problems when I could... I just want a basic idea of where to start, as far as finding and fixing the issue goes. Please help! Thanks!