MKI octavia reverse lamps and sensors not working

[deadtom]

2003 Octavia L&K estate, 1.8T manual, reverse lamps and sensors aren't coming on when I go into reverse.

I am hoping someone here has had this issue before and knows the fix, because I have exhausted the simple solutions and I'm not sure where to focus my efforts next.

So far I have done the following:
- checked bulbs and wiring in rear light clusters; all work, wiring is in good condition, no evidence of leaks from rear washer hose
- replaced control module (second hand part, but from what appears to be a reputable seller)
- replaced reverse light switch on gearbox; contacts and wiring look in good condition, albeit covered in a bit of grot as you'd expect from something in the engine bay
- checked fuse is OK, and checked voltage at the fuse (10.3 V that comes on with ignition, doesn't change when put into reverse)
- checked voltage to the switched live where the loom plugs into the control module (11.8 volts with ignition on), but nothing from the input from the reverse switch
- local garage found no fault codes via OBD
- no warning lights on dash

I believe there is another bit of wiring for this system behind the dash, but there are a lot of wires behind the dash so I don't know what I am looking for. Does anyone know if it could be a problem here, and if so how do I locate the wiring (it's a 10 pin plug I think?) in question and what should I expect to do to fix it?

I've only had the car since August and the system did come on a couple of times soon after I bought it, but there's been no sign of life from it for a while now.

The rear screen demist doesn't work either, which I would have assumed would not be related, but I know electronics can do weird things and affect seeming unrelated systems, I thought it worth mentioning anyway.

Thanks all