OK your note made me remember to look at the schematic
thanks.
Here is the way the reverse lights work: Starts at fuse 7, from the fuse box it goes to S16/1, the starter lockout/backup lamp switch. Power goes in at the #1 terminal (that's a black with red tracer wire) and out on either socket 2 or socket 7, couldn't make sense on why "7" was in parenthesis, either way it would be a gray wire with yellow tracer. From there it goes to N7, which I believe is in the rear of the fuse bx, it's the exterior lamp failure monitor module. This wire connects at connector 1, socket 13. Each reverse light has it's own circuit leaving N7, they are on connector 3, socket 5 and 7. They then just go back to the rear lamp units. Each side has a different ground, so it's not a ground issue (or really unlikely that it is anyways, since they both fail at the same time. I'd concentrate more on the front half of it. You'll almost want to test in a failed state. If you can get it to fail when you want it to, like in the driveway or something, first I'd test for power at that fuse 7 first. After that you need to be under the car and test power at that red/black wire at socket 1, then find out if your gray/yellow wire is at #2 or #7, then test for continuity from 1 and 2 (or 7 as the case may be), then probably easiest to test continuity on the gray/yellow wire from the reverse switch up to that connector on N7 (connector 1, socket 13). Any questions let me know. I'll also go and look through the seat belt presenter diagram.
Gilly