Hi Stefan. I am currentyl looking for somebody with a 1988/1989 Morgan 4/4 because I had issues with a new wiring loom I got.
In theory if the Gomog page is correct this should be the wiring diagram for your car:
Wiring 1988/1989I assume with Spotlamp you refer to additional lights (foglights)?
These would be on the blue-white (UW) cable according to the plan. In my replacement loom they are blue-yellow (which is long range to the BS-AU7 standard) and I believe correct. It is the red-black cable for the rear fog light.
The fuse for this is glass fuse inline behind the dashboard. Look for the red (or red-green....) to red-black cable fuse.
Would you mind sharing a picture of your main fusebox? I am restoring a 1984 - but the new loom I got is for a later model (at least I am strongly convinced that it is)

Regards
Tim