I have also tried to chase down the USDM fog light circuits so that I could use them as DRL's. After reading Trevor's discussion of a possible solution I though i could do it. However I gave up trying to find the locations of the various relays and wiring. So I scrapped that plan and just started over with a circuit that is independent of the existing wiring. The result is that the fog lights can be switched on with the existing fog light switch when the ignition is in the "on" position, regardless of the headlight operation.
The new circuit adds a relay that is powered by the existing fog light switch. The 2 fog lights draw a total of 8 Amps so I used a standard 30 Amp auto relay to handle the current (Radio Shack 275-226 - $6.79). The relay is mounted above the transmission resistor, behind the battery, in a plastic box with the shift-kit vacuum switch.
The schematic below shows the mod with the new wiring in red.
I started by removing the battery so I wouldn't short anything out and to provide access to mount the new relay.
I then popped out the fog-light/cruise/security switch assembly and removed 2 of the existing wires on the fog light switch (white and yellow wires circled in the photo.) The third wire (red) provides power for the indicator light in the switch and is not removed.
In the new DRL circuit the center contact of the fog light switch provides switched power to the fog light relay. This requires running a wire from this contact (circled in the photo) to the relay behind the battery. You can see the 2 insulated wires that are disconnected from the switch.
I tapped into the ignition switch +12V power ("on" when the ignition switch is on) by connecting a short jumper wire (circled in this photo) from the cruise control switch +12V power to the fog light switch L/H terminal. Be sure you don't remove the existing wire from the center terminal of the cruise control switch when the jumper is added. This could require 3 hands!
This completes the wiring modification of the switch. I won't discuss getting the wire through the firewall and out to the relay. I never enjoy working under the dash.
The next step involves making the connection to the negative side of the existing circuit at the fog lights. I broke into the wiring at the fog light disconnect behind the driver's side headlight assembly. This photo shows the location of the disconnect to the left of the headlight bubble level. Above the circle you can see the wire I spliced into this existing wiring. This wire connects to the Red-Grn wire inside the ribbed sleeving and is routed over to the new relay, behind the battery. In the existing fog light circuit this wire goes from the fog lights to the existing fog light relay. Since this existing relay can not be switched (we disconnected the wires from the switch) there is no need to break this wire. Be sure to connect to the Red-Grn wire, NOT the Red-Yel wire which is also inside the sleeving. The Red-Yel wire is connected to +12 V for the fog lights and it is always hot.
The final connections to the relay are shown in the schematic diagram.
This picture shows the complete assembly before everything is buttoned up. I located the relay in a 5"x2.5"x2" plastic box (Radio Shack 270-1803) along with my transmission shift-kit vacuum switch. This photo shows the box and the wiring with the battery removed. To the right of the box is the cover with an attached metal tab that is used to mount the box under the upper Transmission resistor mounting bolt.
Here's the box mounted above the Transmission resistor.
Finally, the installation with the battery back in place.
