LHD W.G (West Germany) wires from dimmer & dash switch to headlight relays?

[Ivan141]

Try removing the headlight bulbs, then measure voltage at the power outputs to the bulbs in order to check your relay circuitry.

I get the feeling you are overthinking these electronics, there is very little in there that could cause any real oscillation. Mostly those kinds of effects can be attributed to low voltage on the relays, causing them to trip on and off fast.

Also , if you're unsure about the switch, connect Red-Yellow switch wire to ground with a temporary toggle switch so you're sure the dimmer relay is being controlled properly.

BTW, did you disconnect the indicator light circuit again? You say you had it working before you added that circuit, so it would be logical to undo that first to try to get back to a working system.