Solar sunroof randomness - any explanation?

[HPsauce]

My ventilation fan seems to be completely random as to whether it fires up from the solar sunroof.
Some days it will be annoyingly silent in blazing sunshine (though today it's working) and others it will be ticking over nicely even when it's quite dull.
The fan itself is effectively barely run in; replaced by MJ the other year with a brand new D3 version (the same just different connectors).

I'm not 100% sure how the sunroof and fan get connected up, though I know there's a relay, but is that a simple ignition-off trigger or something more subtle? I'm sure I read somewhere that the HVAC system is also in the loop.

Anyway, assuming I want it to work consistently, where to start? Replace the relay first maybe, that is presumably cheap and easy?

[steamship]

I'll answer the easy question first. The relay part number is 8A0951253A and is stamped with the number 203. It sits in position 7 in the relay plate in the passenger footwell.

EDIT: Even though I have the full loom for the solar sunroof (I never fitted mine) I wanted to find ITs thread on it, which has a proper explanation of the connectors. If you check the thread, you'll see that there is a single wire that goes to the climate module. The odd thing was IT never showed the second part of the loom that runs to the HVAC unit itself. The only sign of it is in the closeup of the big fuse connector.

https://forum.a8parts.co.uk/showthread.php?t=5370

UPDATE: I also came across the following two lines as to why it may not be working. The first is from ELSA and the second in response to why someone's wasn't working, so not sure about the latter:

if the "residual heat" function is active

middle vents (front and back) are opened