Anytime Fog Light Mod, please help an engineer out

I have a 2019 DCSB MT TRD OR with the tech package. I'd like to mod my fog lights so they stay on when the high beams come on. I'd prefer that they go off with the headlights. That is all I really need.

I was doing some other work on my truck tonight (adding an input to my DC-Dc charger for solar) and thought I might as well do that fog light mod while I have the hood open.

So, I popped off my fuse box lid and found the FOG FR relay and yanked it out. I did the following tests:

1. I started the truck with the low beams on and put the fog switch to on. The end pin on the relay had 13V, then next pin in had no voltage. Of the other two pins (smaller signal pins), one had 13V and the other zero. Ok that makes sense, the two big copper pins carry the load, the smaller aluminum pins drive the coil. I have signal to the coil and power for the load, with that I will turn the lights on.
   
2. Test two, shut off the truck and put a jumper between the positive battery terminal and terminal that didn't have 13V on it and the fog lights come on. Great, that is where I send power to the fogs, the load terminal.
3. I start the truck and put the high beams on, the fogs go off. I yank the relay and check the terminals. Same two were at 13V, one on the supply, one on the signal. Same as before when the fogs were lit! How is that? if that were the case, the power would be going to the terminal that lights the fogs even when the truck is off? I checked this twice. Maybe three times. I even thought maybe the ground is floating on the other side of the coil, but I had 13V across the two coil terminals.

Now, just so you know that I know my way around a multi-meter, I'm an engineer. I used to be the guy that aircraft electricians called when they couldn't diagnose a FADEC on a helicopter (the brain of the engine). I was the guy the who famously changed the wiring drawings to a flight control system in the field with a red pen and got it approved for install.

Any ideas how it appears that this relay has the same power on brights and on dims? Is there another relay between this one and the lights that do the high beam cutout? I can't figure out how to complete the mod without understanding why the relay terminal meter out the same with the fogs on and off.

Any idea what a good source would be for power to drive the FOGs to only get supply power when the lights are on? Would Towing Tail be a good candidate, that should come on and go off with the headlight.



Access to my fuse box is a pain, you have to unscrew the one end of the shelf that I have my DC-DC charger and battery tie switch mounted on. You can get to the fuse box, but it isn't as easy as I would like. I'm pointing at the relay in question here:


Any help is appreciated!!
-Chris.

 

Here’s the wiring diagram for the fogs.

 

@cludwig

What you're looking for MAY be found here:

3rd gen tacoma anytime fog mod!!!!!!!!!!!!!

No guarantee!

 

I was afraid of that. They made the mod a PITA for gen 3. Summary of the mod is to bypass the entire wiring of the car. No thanks.

Thanks for that link!!

 

You would have to tap in to the hi beam wiring as well as run another wire to the battery to be able to make it work the 3rd gens are programmed to not allow the fogs to run with the hi beams and from my experience that’s the only way to do it to by pass that

 

NO need for another wire to the battery...

 

If you tap into just the hi beam wire you won’t be able to run it both because the hi beam wiring is limited to 15A which most likely won’t sustain both sets of lights running

 

I think you can tap into the high beam as a trigger to the fog relay. But I'd use a diode so it doesn't back feed