Don't do it the way you're suggesting.
On the driver's side, make an H13 to H13 dongle and run an extra wire from the lowbeam hot to trip your relay. Mount a separate switch in the cabin so that you can select either set of fogs to run ---- or both if you're offroad.
As was stated, it's illegal to run both at the same time onroad. Or to have them switched together and used onroad, etc.
The dongle will need a capacitor to keep the relay from buzzing to death. A diode is also a good idea to keep the power moving in one direction.
If all of this sounds too deep for you, call Ray at rallylights.com and ask him to build the little dongle for you.
You could even buy one of their Hella headlight kits and have him put the extra wire on the harness for that...
Originally Posted by OH9JK
You will need to use a relay for the aftermarket lights. Tap into one of the hot wires off the OEM lights to trip the relay. Make sure you use proper gauge wiring and a fuse!
I'm not sure if the foglight circuit is PWM'd or not, but you really shouldn't have the lights daisey-chained that way. If he's got a relay and switch on the new light circuit, at least he can defeat the new lights. But he still can't run them unless the originals are on also.
And if the circuit is PWM'd, the relay will die without the capacitor.