@alfa01,
I think if you pop one open you might be surprised at how easy to repair that generation of circuit boards are. Especially vehicle pcbs as they have to be "rugged".
The power caps and ICs are the likely culprits. Check the solder points underneath the pcb too (most pcbs of that era are through-hole). Look for the solder to be cracked away from the via. A crack will look like a clean ring around the entire joint.
Heres an example off the wiki: [https://en.wikipedia.org/wiki/Soldering#/media/File:Gebrochene_loetstellen.jpg|soldering]
The other thing to look for is blown capacitors. The electrolytic will ooze out the bottom (the top if they're really bad) and you will see gunk around the base of the cap. The bigger they are, the more likely a suspect.
Also, take some pictures and maybe we can help to trace out the obvious circuits to help narrow things down.
Cheers