I discovered my my front seat heaters were non functional after purchasing my XJ8, which traced back to the rear fuse box and one of the infamous "system relays".
Obviously it had been an issue for a while as the prior owner (or their auto sparky) had bypassed the troublesome relay as it also fed a supply to some of the rear light cluster functions.
So there were multiple additional wires added in the rear to cobble some functionality back to the rear lights as a minimum - no doubt to get the car through a warrant of fitness.

So as Pete said, have to remove fuse box, split the two PCBs apart to then replace the relays - I say relays as I suggest you just replace all of them to eliminate any future problems while you are in there!
You really don't want to be repeating the process by taking a short cut.
Note: You'll need a pretty powerful soldering iron as those heavy copper tracks disperse the heat efficiently, so a 25W iron just will not get the job done.
I ended up doing some careful preheating with a heat gun to assist my Weller iron.

 

Hello dear Fellows
As mentioned further up, I had those weird behaving rear lights. I was able to get a replacement Fusebox (used) and installed yesterday night. And I'm happy to say: The light-mare ended, the lights work perfectly now, also both reversing lights now work.
Was not able to check the driver's seat heater yet, to much snow for the XJ at the moment but will let you know.
I will have a closer look at the defective fusebox later - I don't know anything about the history of the used replacement part.
Regards from Switzerland, Beat