1982 Gmc electrical mystery-Really dumb question

[gregg72340]

Ok, this one is going to sound really stupid but here goes. This truck has been sitting for the good part f 10 years and we are going to try and get her running again. Everytime somebody needed a part for their other trucks we would steal it off of ths truck. Now we cant remember what one part was that we took. It was bolted on the firewall and it was a small square black box. This is the last piece we need to throw back on her to give her a shot at life again. Definitly electrical part we remember that much, could it have been a voltage regulator??????? any help would be appreciated