Hi David,
This morning I hooked the operating unit up and checked this out, (holding the rest and defrost buttons simultaneously) but nothing, I performed this check both with the car running and stopped with key installed.
Though It sure would have been nice if the operating unit had it own self diagnostic ability.

Can you suggest anything else ?

Has anyone had to have one of these units repaired ? or are they simply thrown away and a new one installed ?

Thanks for you reply David.