Solar sunroof intermittent now not working

[HPsauce]

My question is, where do I start on diagnosing and fixing it?
The ventilation fan works totally normally except the solar sunroof no longer activates it.
For quite a while it had been intermittent, sometimes working, sometimes not, suggesting an relay or loose wiring issue.

I know there's a lot of stuff on the A8parts forum that could/would guide me but it's never up these days!
Where's a good place to start, checking voltages etc.? Or even just swapping a relay?