Originally Posted by willwren
On my car, the fogs are controlled by the adaptive light control module. Tricking that would be tricky.
Just thinking out loud here, you could do a workaround via a 5-pin relay that would allow the ALM to remain undisturbed.
On a 5-pin relay, the 5th contact is a normally-closed one. That is, when the relay is off, there is still a path through to one of two output contacts, so you can have one circuit closed at all times. When the relay is energized, the main circuit is redirected to the other output contact instead.
So you can, of course, turn this around and have the relay selecting between one of two power inputs, depending on whether it'* energized or not. The foglight lead wire would be connected to what would be the common output terminal of the relay.
When it'* off, normal foglight operation can occur; when it'* on (such as if it was tied into the high-beam circuit), it can swap in a different power source, so that the foglights can come on with the high beams.
Clear as mud? I thought so!