I haven't done it, but the solution depends on the exact behavior you want. The discussion below assumes you have a U.S. version 4runner.
As designed, the fog lights are on IF AND ONLY IF ALL of the following are true.
1) Fog light switch is in the on position
2) Dimmer switch is in the head light postion OR dimmer is in the auto position and it's dark outside.
3) Hi Beam switch is disabled
4) Hi Beam flasher switch is disabled
5) A bunch of other junk (key inserted and turned on, battery installed, fuses installed, .....)
I assume you want to add another possibility to item #2 while leaving the other requirements as-is.
2) Dimmer switch is in the head light postion, OR dimmer is in the auto position and it's dark outside, OR dimmer switch is in the tail light position.
Is that correct?
If so, what you need to do is modify the input to the front fog relay. The fog relay coil (pin 2) is driven by the switched output of the headlamp relay. You want to disconnect this wire. You'll have to figure out how best to do this.
Next you want to connect the switched output of the tail lamp relay (pin 3) to pin 2 of the fog lamp relay.
That's it! These instructions may not work and if followed could result (and almost certainly will) in bodily injury or significant property damage. Your truck may start on fire and the entire universe may be vaporized. You do this (and any other) modification at YOUR OWN RISK. If you don't accept the risk, please ignore anything you've read here.
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