Bottle dynamo wiring a headlight and taillight on 1980s bike
@Plasma says: I got it working! You're right @Carel, the frame itself wasn't negative, but the back fender somehow was. I didn't check if the entire frame was positive, though, so can't say anyrhing about insulated wires or not. Anyhow, the problem seems to have been a missing connection between the positive and the taillight bulb. I DIYed a new socket for the bulb and did some (possibly redundant) new wiring to connect everything properly.

(because I don't like unanswered questions)