I have a none working fog light on the rear right tail light. I've got 4 bulbs that all are working on the left side, so it's not the bulb(s).

 

Are there anything special about these sockets and/or which H21W/BAY9s socket can I replace it with?

 

PS: I cannot see anything regarding wires or connectors that look blatantly wrong, but as I have not tested them I cannot say for sure. I thought it would not hurt to just find a new socket and try to swop it out and see where that leaves me.

It's normal that only one rear fog is activated from factory.

 

On a RHD car, it would be the right rear and on a LHD (like yours) it would be the left rear

Uhm... What's the purpose of that?

 

 

Regulations only require one fog light so only one is enabled by default at a guess....? Some folk claim it's one fog to avoid it being confused with brake lights, I've no idea of the real reason.

 

However, it's cheaper from a manufacturing stand point to only have one set of rear lights to cover both LHD and RHD. Therefore you end up with a unused bulb

Saving the cost of a bulb and the effort of fitting it is a worthwhile saving for the manufacturers when building thousands of cars. Many companies employ engineers to implement cost savings. I have had that role at times in my career.