Our 2012 Pop has been complaining about a blown bulb, the warning only comes on when you first press the brake pedal, so it was one of the brake light bulbs.

After messing around on my own trying to work out which, I pulled the light cluster and stripped it down.

There are three bulbs in each, the middle one the indicator, but oddly the top and bottom are both twin filament P21/5w bulbs.

Now I was certain the bottom of these was the tail and brake light and the top just a tail light, so I went and checked the Mother in Laws Panda and I was right.

It's actually marked on the metal circuit inside the cluster with a stamp in the metal, both top and bottom are marked P21/5w, yet the top circuit only has the one contact for the bulb
(which is off centre to contact just the one side of the twin filament bulb)
and in the handbook it states Rear Side Lights Type P21/5w, Power 5w. (and Brake Light P21/5w, Power 21W)
Now why has the top bulb got a twin filament to it?

It saved me a trip out to get another bulb as I just swapped the top and bottom over, but I found it very odd.

I noticed this too, and wondered why. The only thing I could think of is the part that holds the bulbs can then be used on either side of the car (one way up or the other), but still the actual contacts need to be fixed in a way specific to each side. Usefully it limits the number of types of spare bulb you need to carry in the glovebox to just 4 (we all do, don't we?)... the 21/5w is then used for the DRL, tail and brake lights. The only others you need are and orange indicator bulb (same at both ends of the car), plus a number plate bulb one for the headlights. Oh, and the rather less common 16W big wedge ended bulb used in reverse and rear fog. But they tend to last for ever.

My guess was it might be because of "other markets" that have double tail and brake lights fitted (both top and bottom act as tail and brake), where ours has two tails (top and bottom) and one brake (bottom).

If had me going for a short while, once the cluster was stripped it had me thinking why the top brake light wouldn't work even after I've swapped the bulbs.

It was only when I went and checked the rear lights om the M in L's car that the penny dropped and the brake lights are just on the bottom bulbs.

When looking at the metal "circuit" part of the bulb holder, it's clearly designed to take a P21/5w at the top yet only powers the 5w part of the bulb as the single positive connector is off set to only contact one part of the bulbs base.

There's a stub that looks to all as it's broken off (but isn't), but that contacts the the outer, negative side of the bulb.

It all looks very odd, I can see this causing a meltdown of your average Halfords bulb jockey!