I have a 1980 VW Dasher with its original speakers but no radio. It has an unusual (erroneous?) electrical configuration where most accessories (e.g. lights, cabin fan, etc) still run even with the ignition switched off and key removed (I've heard that's just how old VWs work, but never confirmed that). This results in the problem that installing any aftermarket radio or amplifier will run the battery down if I connect it in the conventional way. So 2 related issues here:
1) I don't really want a radio. I'd actually be just fine with some sort of simple, low-power amplifier that can drive the speakers, which I could plug a smartphone or something into (like with a TRS connector or whatever) to play music and whatnot. Has anyone had any success doing something like that? What kinds of products should I look for? Right now I don't even know what such a thing would be called. "Amplifier" encompasses too many different products.
2) I need to put my own toggle switch into this system somehow, so I can switch the radio/amp off when I'm not using it, and avoid running down the car's battery. How should I approach wiring this? I'd like to avoid breaking out any books on Electrical Engineering right now, and hopefully just accomplish the mission with the bare minimum of knowledge (I'm assuming the people who install car stereos don't necessarily know everything about voltage and current, so hopefully I don't need to either). In the absence of specific suggestions, links to more general guidance about doing this sort of thing would also be helpful.