I do not know the answer and, not having considered a Y C70's wiring diagram but, I assume that the footbrake circuit/lights are potentially 'live' all the time even when the iginition if off. It just takes a push on the brake pedal to make the circuit to activate the brake pedal switch and stop lights. At least that's the way it works on my '98 V70.
The brake pedal switch is a complicated little 'gem' (requiring set up) being off when when the switch button is not touching the pedal when the brake pedal is not operated.
When the brake pedal is operated the pedal arm moves down making contact with the switch button so making the brake light circuit.
I would be looking at a common earth connection between the head unit and the stop lights for the fault.
Bob
