Someone will have and be able to interpret the D3 wiring diagram; it sounds to me EXACTLY like the relay isn't switching.
That of course could be a faulty relay or a failing trigger signal (or wiring).

The solar panel is connected directly to J126 blower control unit. That appears to have a single connection to / from J255 climatronic control unit.

http://forum.a8parts.co.uk/attachmen...1&d=1568747638

So "almost, but not quite, totally unlike a D2". I guess V2 is the blower motor and J355 the solar panel?
What is C20?

C20 looks like the solar panel to me, judging by the symbol. So I expect J355 is a DC-DC convertor (probably stepping up/down the solar panel voltage to provide a consistent, regulated output).

Indeed, I didn't look properly at the C20 symbol. Another level of sophistication over the D2, so probably almost nothing to learn from my/our comments I'm afraid. :(

Yeah C20 is the panel, J355 is the regulator.

J126 is the part I replaced, but the replacement I got was a used one from a car of similar age.

It's quiet possible that the used one is also faulty, so I might go ahead and buy the new no OEM one from ebay.

J126 is what controls the fan speed, it has a number of inputs, and just two wires going out to the fan motor itself.

The fan is definitely running on solar power when the engine is running, you can see it spinning quiet happily by looking into the box with the pollen filter removed, ant it's speed varies based on the amount of sun.

There is 12V coming into the controller from the car side, and the voltage from the sunroof varies based on the amount sun.

It is possible of course that J255 is faulty, in that maybe the required signal is not getting to J126. I have no means of checking this as I do not know what the signal that controls the speed of the fan into the controller is meant to look like. J255 has component protection, so it's not easy to just swap a replacement in.

From the SSP it looks like J126 is on the LIN bus so tricky to see what J255 is telling it.

I found a document explaining the LIN bus, I guess its working because I don't have any fault codes except the implausible signal from J126. I do not have the no communication fault mentioned in this document.

http://www.volkspage.net/technik/ssp/ssp/SSP_286_d1.pdf

I think it's safe to assume that default mode for J126 if it does not receive signal is to switch to solar for example when ignition is off and climatronic is switched off. So it looks like J126 in not seeing signal from J255 either because it is itself faulty and does not sense signal, there is break or short in wiring or J255 is faulty and not sending signal. You could start checking for break or short in wiring with multimeter and if you have oscilloscope you could check if there is any signal coming out of J255 and if it is reaching J126