Have you thought about trying to find one of those old electronics laboratories that Radio Shack and the like used to sell? I'm not sure if anyone makes them anymore. For most vintage stuff, you're going to need to understand switches, relays, potentiometers (fuel gauge) and motors, plus of course how electricity flows.

You might also check out hobbyist electronics, such as https://learn.adafruit.com/ and https://www.sparkfun.com/ - They (and tons of other companies) sell DIY kits that you solder together. Not only will you get an idea of how the electronics work, but you'll also get lots of practice soldering. Get yourself a decent variable temperature soldering iron and solder, some helping hands, and a few kits that seem interesting, and go to town.

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild