So here is the official requirement for rear fog lights in Poland. Poland is a member of EU and most requirements are the same across the Europe, so it is safe to say, that if the car meets Polish requirements i will be good for the whole Europe. This is the EU regulation "ECE-R38" and "ECE-R48":
1. If one rear fog light is used it should be placed on the left side (right side for right hand dribing like in UK). It is allowed to place one rear fog light in the center. If two lights are used they must be placed symmetrical on both sides. It has to be placed no higher than 100cm above the ground and no lower than 25 cm above the ground. If rear fog light is used as a part of the tail lamp the upper limit is 120cm (4runner is good for that).
2. Important: rear fog light must be at least 10cm away from the stop light. I'll explain the problem later.
3. Color must be red, the brightness must be similar to stop light 150 - 300 cd (equivalent of 20W bulb) and must face rear +/- 25 degree left/right and +/- 5 degree up/down.
4. Must have its own switch (separate from the front fog light switch) with light indicating "On" position. Can be turned on only when headlights or front fog lights are on.
Because it is allowed to have only one reverse light (ECE-R48 and ECE-R23), the simplest way would be to replace the white reverse bulb in the left lamp with the red one (incandescent or LED) and route wiring to separate switch. But because of requirement 2 and the fact that reverse light is next to stop light this would not work. First you would need to swap positions of reverse and turn signal bulbs, so the reverse bulb is on the bottom (or top depends on 4Runner year) of the lamp and turn signal in the middle, then you can use this reverse for rear fog swap solution. I did that to my Nissan Versa Note and for 95 Tacoma.
This solution is quite popular in small cars but it sucks as you will lose one reverse light. I'm still searching for a solution for my Tacoma - the best way to add either reverse light or rear fog light.
In 4Runner I did custom lamps that I placed in the rear bumper trim (the rear fog lamp building thread for my 4Runner is here
http://www.toyota-4runner.org/5th-gen-t4rs/204372-2012-sr5-rear-bumper-question.html). Looks factory, is legal and 100% functional.
