BMW Series 1 model year 2014

A couple of weeks back I tested a series 1 and visual inspection of the tail lamps showed nearside very bright and offside dim. The nearside looked like a stop lamp operation and at that time I took it that it was too bright and advised it. However, now I'm not sure which tail lamp is actually at fault?

Initially after removing the lamp clusters I changed the standard 21W tungsten bulbs because they were darkening on the glass. This was the time I realised those bulbs were actually the stop and reverse bulbs. Anyway then realising that the lamp units have intergral LED tail lamps that are sealed and part of the lamp, and they will more than likely be expensive, I'm not sure which lamp if any are the cause of the fault?

According to the scanner live data the voltage supply x 4 individual readings read approximately 15V each except one that is static at 12.00V The scanner does not make it clear what those four voltages are supplying. Referring to the wiring diagram the lamps are supplied from the body control module, but only two voltage supplies, and two links to the lighting control module for what looks like a supply from the body control module. 

I suppose the only test I can do is check the voltage logic level to each LED tail lamp and see what voltages are recorded. If then I have the same voltage level to each lamp (and a good earth return of course) then I can conclude the fault is in a LED lamp unit.

Anyone come across this fault on BMW's and ever fixed it?