Rear light clusters can suffer from bad earth connections in three places. If only the dual filament circuits are affected then the problem is most likely between the bulb and the holder. Arrowed is the blob of solder at the top of the bulb base which can be used as a test point. If you see anything more than 0v here with the bulbs powered then there is a bad earth:

If, with the bulbs powered but not working as they should, you have an earth at point 'A' (test against a known good 12v source) then a bad connection between bulb and holder is the likely cause. However if point A shows any voltage, then the problem is more likely to be where the light unit is attached to the wing, but that should be affecting both bulbs i.e. all three filaments. This would show up as a voltage anywhere on the base of the light-unit 'B' and the ends of the fixing screws. I have heard of voltage being seen at A, but not at B, only the dual filament bulb was affected, and this was caused by a bad connection between the bulb holder and the base of the light unit. About the only thing you can do about that is solder a wire to A and terminate it under a nut on one of the fixing studs, on top of the standard 'nut':

The dual-filament stop-tail bulb holder. The channel is deeper on one side than the other to accommodate the off-set pins that should exist on these bulbs to ensure they are installed the right way round:

The special pressed DOTLOC 'nut' BHA4242 that has spikes (arrowed, three on each) to make electrical connection to the wing:

Oddly, despite the spikes to cut though the paint (arrowed), there is a plastic washer on the back of the nut:

NLA from the usual suspects (but seeming available here) replaced typically by conventional nut NH911011 plus star washer WE702101. Take care not to overtighten the nut or you may pull the stud out of the light unit!