Hi,
I think I remember the answer to this one from the past.
Volvo fitted exactly the same lamp cluster to XC90 pre facelift irrespective of which market they were being supplied to. The side/marker position contains a twin filament bulb on both sides.
European legislation requires a high intensity rear fog lamp to be a minimum of 100mm from a brake light, hence why we have the rear fog lamp in the bumper on the driver's side and the opposite fog 'lamp' is just a blank reflector assembly. The main rear lamp cluster just isn't big enough to meet the 100mm rule.
America is different, from memory on a USA spec car the side marker lamp also becomes the rear fog lamp when activated, hence the twin 4w/21w bulb, and both of the reflectors in the rear bumper are dummy lamps with no wiring to them.
I imagine that the wiring or module control for lighting is different in USA & some other markets, illuminating the 21w filament when the fog lamp switch is activated.
No doubt someone will tell us I'm wrong, but that was my understanding when I delved into it years ago!
Paul
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