I know the yellow wire in C1 D3 of the underhood fuse panel will maintain your Low beams while you switch to highs if you ground it out. What i am wondering is if you can diode from the purple wire in C1 C3 (headlamp high beam signal), to the Dark green and white wire in C1 B3 (Fog lamp relay control), and have your fogs come on with your high beams. I know its been done with relays under the dash, but i know there has to be an easier way.

what i know:
The Purple wire is switched to ground when the high beams are on.
The Dk Green wire is the wire to trigger the relay for the fog lights, and it is switched to ground to trigger the relay.
In theroy a diode would work, but how big of a diode would I need to handle the draw of the high beams, and what direction does it go? I am assuming the Stripe goes towards the Purple High beam wire.

PLEASE DO NOT TELL ME HOW TO WIRE AN 03-UP, IT IS DIFFERENT. THERE IS ALREADY 100 THREADS ON HOW TO DO THAT.

hi there sorry can't answer your question but currious about what your saying about grounding the yellow to keep low beams on when on high beam never heard of this is it really as easy as you say?

Yes, it is easy. Just look behind the headlight, find the yellow low beam wire and ground it. Only for 99-02 GM trucks.

On the 99-02 trucks yes it's that hard, I only say that cause it's even easier on the 03-07 trucks. Yes it's as easy as a scotchlock tap and a short piece of wire. If you search this forum, there are pictures. I don't use that method since I have a booster harness.