Those cars had no relays in the lighting circuits so the load on the switches was very heavy. My '92 Plus 4 burnt out the main switch several times and the dip switch a couple of times including once when we were in Norway with no chance of a replacement. The car is probably also fitted with a dim dip unit which can throw in some strange failures although that was what meant I had some lights when the dip switch failed in Norway.
As you have no side lights the main switch is the most likely as Chester.G suggested. I don't thing they are affected by either the dip switch or the dim dip unit.
GoMog has a wiring diagram which may help.
http://www.gomog.com/allmorgan/wiring1990on.htmlIf you decide it's the dip switch there are several variants with slightly different connectors. I used a Morgan supplied part for mine as at the time you could still order direct from the factory and even then I had to change the connector slightly.
Good luck
Dave
