I need to tap a wire that give +12V when the headlights are on to connect to an aftermarket radio in my 04 XC90 (to dim the display when the headlights are on).

Do I need to tap into the engine compartment fuse #17 or 18? Is there an easier place to tap inside the car?
Thanks.
-Carlo

 

You could use +12V constant and tap into one of the switches that light up when headlights are on -use switched low current to switch a regular bosch relay

 

Thanks... which switch lights up when the headlights are on?

 

Use the wire from the headlight switch.