So essentially the first problem is being too posh having a heated front AND rear screen
I'm not too jealous.
Looking at it I think there are 4 items to test, essentially the 2 switches and the 2 relays.
So looking at how that should operate, the first test is to check there is 12V going to the red wire and that it comes out of all 4 connection points.
Check 2 is going to be, with the brown K wire disconnected that when you pull the switch on that 12v's crosses the switch and comes out of the terminal that the brown K wires connect to.
That will then tell yo that you have 2 working switches.
I cant say I have seen those weird relays before, I guess I wouldnt have wired them that way and just used 4 pin relays, turning them off when they are clear ( I know on my golf its like a 15 minute timer) but my guess is that the brown wire activates the timer.
So next test it to turn on the switch and see weather the brown wire has power coming out of it at the terminal, if so, is there power at relay terminal 3 and 4 finally resulting in the test for output at terminal 5.
If the answer says yes to power at those terminals, in theory it should work.
So the next test is going to be the connections actually at the screens, making sure the Earth is properly earthed and there is a supply power.
With rear screens, you would have had to completely gone through all tracks for it not to work, but you should see a resistance value if you connect a multimeter with the supply leads disconnected, part of me wants to say its something weird like 86ohms it might be less or more.
So give some stuff a try and see where the power stops.
Do you have a part number for the relay? Its rare but occasiaonly the timer can go, as essentially its a similar relay to glow plugs and they can go, often its the timer circuit, not the actual relay part.