I had a drivers window control switch (DDM) that misbehaved in somewhat the same manner - OK with the key at position 1 but acted up with the engine running. A replacement switch was the ticket.

You say you tried replacing the switch? Are you confident you installed a good one?

Voltage fluctuations will wreak havoc in P2 Volvos. Get that sorted first. Good battery and well performing alternator. I suspect you have bad diodes and ripple current on the alternator.

Engine running is a different status on the network. The SWM(steering wheel module), the plate the stalk switches mount to, could be going offline and then the wipers will default on for some P2s. Have you noticed if turn signals function or high and low beams can be selected when wipers are doing t...