Initially I noticed from carista that the rear left sidelights weren't working, they weren't, I try them about 15mins later and they are working, a few days later I notice that the rear left brake light isn't working also the side light half work or at least the bulbs are dim, I pulled them out to look and compared to the right side bulbs. Also I pulled the brake light and cluster bulbs out and they don't look burned through though the brake light filament is darkened a bit, that may just be wear and tear or the sign of voltage fluctuations.

 

Obviously an electrical problem like a bad connection or fuse.

 

Where would you start with this?

 

The right rear side if fine.

 

I would start by looking at the earth for the rear lights which is a stud on the body in the boot on the near side.

Corrosion caused by screen wash or water in the boot area is not unknown. 

 

If that is not the cause I would take out the left rear light cluster and check for poor connection in all parts - bulb connectors, wiring connectors, earth connections and for corrosion/dirty contacts.

Dim lights is usually caused by a bad earth connection. What happens to the dim lights when someone activates the brake lights on and off?  If it changes in any way suspect an earthing issue.

 

Finally, check the solder pip on the end of the bulbs for reduction in size caused by fretting. Either build up the solder again or replace the bulb.