This mod will turn on the fog lights with the high beams. If already on, they will stay on with high beams.
I did the Fog mod a while ago. The green wire in the pic goes to the aux fuse in the in-cab fuse block, allowing fogs to come on anytime I want. But what I really wanted is the fogs to come on with the brights. A little reading showed me the light. The Fog switch connects the ground (ground switched). From what I could find, the headlight and dim relays are positive triggered, so a simple diode bridge to the fogs won't work. So I connected a relay to the negative terminal on the Fog relay, and triggered it by the bright positive output terminal of the Dim relay. Ran the wires out the bottom of the fuse block to the relay. In the pic:
1. First release the fuse block, approx catch locations are shown in GREEN
2. Feed 2 wires down into fuse block through existing holes (magenta arrows), out the bottom where wire bundle exits at rear (blue arrow).
3. RED arrow to the left points to + bright out terminal on the DIM relay. I simply shoved the end of the black wire in the pic in the slot and replaced the relay. If you want, verify that this only has 12V+ when the brights are ON. This wire goes to pin 86 of my relay.
4. RED arrow near center of fuse block shows where I shoved the blue wire into the - (ground) trigger of the FOG relay. Replace the relay. Wire goes down the hole shown in magenta and out to pin 30 of my relay.
5. Make sure both wires are firmly placed and won't wiggle or come out.
6. Jumper pins 85 and 87 on the relay and run a wire from these to ground.
7. Test everything. Fogs should operate as they did before, which will depend on other mods done. BUT they should also come on with the high beams even if they were off, and will stay on with the high beams if they were already on.
8. Cover wires and place wires and relay in a safe location. I ran mine along the wire bundle exiting the fuse block, and zip tied the relay in the corner by the firewall.
The other fog mods have the problem that if fogs are on in daytime, as it gets dark, they make it look like your headlights are on, and you drive around with no tail/marker/head lights at night. Dangerous and you'll get a ticket. So my next project will be to add a Directed Electronics 545T Nite Lite module (DEI 545T may get more search results). Around $45 online. Unit has a photo sensor and makes your headlights and tail lights auto on at dusk, off in daylight. Can also optionally be wired to turn tail lights on with the wipers.