From the manual:-
Rear fog lamps;
1. They must be operational
2. The correct number must be fitted to the vehicle (Table 1)
3. They must only emit a red light
4. The rear fog lamp(s) must only illuminate when dipped beam, main beam or front
fog lamps are lit (see alternatives in Notes 3 and 4).
5. They must be positioned correctly to meet
a) the positional requirements of Table 1
b) the angles of visibility requirements of Table 1
6. Must be fitted with an operational “tell-tale” lamp (non-flashing) visible from the
driving position
7. Either the switch or tell tale must clearly indicate that operation relates to the
Rear Fog lights (see Note 1).
8. Must not be operated by a brake control
9. Fitted so that the reflector is facing squarely to the rear
10. An optional rear fog lamp must form a matched pair with the obligatory lamp.
11. An optional rear fog lamp must only operate with the obligatory rear fog lamp
12. All rear fog lamps must display an “e” or “E” mark with a “B” or “F” (see Note 2)
You can ignore the options in Notes 3 & 4 - these have led to much confusions and people selling magic boxes of tricks which you do not need, in order to satisfy
optional arrangements.
Basically, you need:-
E-Marked lamp lens
One fog lamp at rear, on the offside (see positional requirements table below)
Warning lamp (telltale) in the vehicle when fog lamp illuminated, with the lamp bearing a foglamp symbol
Clearly identified or labelled switch
View attachment 101205
If it was me, I'd take an extra feed from the third position of your off/sidelamp/headlamp switch, which normally goes to your dip/main switch. Take this extra feed to the input side of a normally-open relay, so that when your lights are on, the relay closes. From the output side of this relay you can run to the fog lamp switch. The result will then be that you can only switch on the fog lamp when the dip or main lights are on (but not when sidelamps are on not when the lights are off. I've made it sound complex, but it's really not too hard (and you don't need a magic box of tricks).