Replaced all my taillights, back up light bulbs yesterday. I noticed an odd thing. Two of the 2 taillight sockets require a dual filament bulb (21CP/5CP) but only have one contact in the socket for the 5 watt contact? The pins are offset on the bulb so only a dual filament bulb will insert and the contact is offset. Whats the reason?

 

That's an excellent question; I was wondering the same thing myself. Maybe it's so you would have a quick spare in case of burning out the brake light filament on the two-contact socket? I've been trying to figure out a way to add the brake contact in the single contact socket...