Technical Debugging an issue with some sockets in the rear light cluster (passenger side)

[Flamekebab]

This afternoon I pulled out the passenger side rear cluster on my 2012 Panda to replace a dead bulb. No problems there - I decided to replace both of them at the same time just to be safe. Once I reassembled things I discovered that neither light was working...

I suspected the bulbs and dropped them into the indicator socket just to check - no problems there, they both work just fine.

Breaking out a multimeter I could see that one of the sockets was getting 5V and the other doesn't seem to be getting anything. The indicator socket gets the expected 12V.
Weirdly there was a brief moment, once, when both lights started working but I was never able to recreate it. Rechecked the bulbs in case any of my tinkering had somehow torched them but no, still working fine. The connection between the connector and the light cluster is also solid.

I'm guessing the issue is the wiring loom as a result but I'm not sure where to start on checking that. Any advice or should I get it looked at professionally?

 

[Panda IIs]

This afternoon I pulled out the passenger side rear cluster on my 2012 Panda to replace a dead bulb. No problems there - I decided to replace both of them at the same time just to be safe. Once I reassembled things I discovered that neither light was working...

I suspected the bulbs and dropped them into the indicator socket just to check - no problems there, they both work just fine.

Breaking out a multimeter I could see that one of the sockets was getting 5V and the other doesn't seem to be getting anything. The indicator socket gets the expected 12V.
Weirdly there was a brief moment, once, when both lights started working but I was never able to recreate it. Rechecked the bulbs in case any of my tinkering had somehow torched them but no, still working fine. The connection between the connector and the light cluster is also solid.

I'm guessing the issue is the wiring loom as a result but I'm not sure where to start on checking that. Any advice or should I get it looked at professionally?

Brake light switches can be an issue but normally throw up allsorts of (seemingly) unrelated warnings. Have you tried swapping the light units (if you can plug into the loom)? You don't have a tow bar fitted do you??? The installation may not be "good"?

 

[Flamekebab]

No tow bar, no.
I've not tried swapping the clusters - I'll give it a go and report back!

 

[Herts Hillhopper]

@Flamekebab The bulbs in the red tail lights have two filaments - a bright 21w one for the brake lights and a dim 5w one for the night time tail light*. This also means they have two ‘positive’ contacts in the base. To be sure they go in the right way round, the pins on the side that engage in the fitting are at different heights and only fit one way. The indicator bulb has only one filament, just one base contact, and parallel pins. So - you should not have been able to put the brake/tail bulb into the indicator lamp holder!
This means: either you forced it, or are using the wrong type of bulb to replace the brake/tail light.
(*Note: only the bottom bulb in each side shows when braking but - for some reason - both top and bottom red lights have twin filament bulbs. Both top and bottom light up as night time tail lights).