I guess we shouldn't be writing here in the Welcome forum, but a quick reply.
I think yesterday I solved the problem. It seems it is the b*y DimDip module misbehaving - there is thread on here all about them and they seem to be nothing but trouble. The particular module here was not like the one described in another post I saw (i.e. not a resistive unit) and the wiring did not tie up with a "series/parallel" unit I read about. Worse, the car harness did not accord with a wiring digram my friend downloaded from GoMog...
No matter, I worked out which two wires from the unit to unplug from the fuse panel to remove power, and suddenly everything seems absolutely fine again.
The still part-wired unit is dangling under the dash. If all stays well for the next couple of weeks, he'll trace the wires back from the unit and disconnect them from the wherever - and then throw the unit over the nearest hedge!!
But may I thank you for the offer of help - much appreciated. And my '74 +8 doesn't have anything so fancy as a DimDip module, nor even a fuse panel!
