There is an earth from the gearbox to the chassis i would open that up and clean it first
It is much simpler to run an additional earth in the engine bay - that’s what I did when my 1985 4/4 had this problem. The factory fitted earth strap at the gearbox is puny. If it is an earth problem, the heater valve control cable could be hot enough to burn you so be careful. Don’t ask how I know …..
