I took a look at a wiring diagram for a first gen R65 .
Both rear turn signals, brake and tail light all have a common connection for a ground .
The junction for this, is somewhere between the tail/ brake light and the connector under one of the side covers, don't know which side this is on, I have a second generation R65 and the electrical system is different .
Don't know you're electrical experience, but you can disconnect the connector and then check for continuity, or resistance reading from there to the metal contact that the outer base of light bulb touches .
You can do this for each bulb, possible corrosion, broken solder joint / wire .
The solid brown wire is the ground wire .