I might have to nick that idea!
Cruise wasn't an option on TD's so the indicator light is replaced by the glow plug light.
The Cruise switch position is a mirror image of the Mk1 foglamp switch position, so a foglamp switch should be easy to get hold of, and fairly simple to modify and fit.
It might be possible to pull an instrument binnacle apart and use the tell-tale graphic on the front of the foglamp switch.