I have to guess at this because of the limited details you put in, but I'm assuming the other end of the yellow wire goes to the 86 pin on the relay? If that's the case, then it's the low current 12V source that energizes the relay to allow the high current 12V power to go to the lights. You have to decide when you want to be able to use your lights. If you connect that yellow wire to the low beam 12V wire, then you can only use your IPF's when your low beams are on. If you connect that yellow wire to the high beam 12V wire, then you can only use your IPF's when your high beams are on. If you connect that yellow wire to the parking light 12V wire, then you can only use your IPF's when your parking lights are on. If you connect that yellow wire to a switch then through a fuse directly to the battery, then you can use your IPF's anytime you want just by flipping the switch.
Keep in mind that whatever option you use, you want to install your switch in that line, between the 12V source, and the 86 pin on the relay. Good luck.
