Originally Posted by 1bmw1
Looks like it would be either #4 or #5 based on the link you provided. The best way to check is engage the brake (have someone sit in the car, or put a heavy object on it) and check out the rear yourself and see which bulb is out. Then just replace it.
Yeah, but I'm not sure which one of the bulbs in the assembly is for the P21/4W bulb, assuming there's only one per side. Picture doesn't really show that clearly.