C4 '90 - RPO C68 Electronic Air Conditioning

[Spinman]

I've been having trouble with the push buttons on my HVAC control panel (1990 / RPO C68 Electronic Air Conditioning) working properly for the past few years. Intermittent ability to access and change fan speeds, temperature setting, and HVAC modes. Most times, it will stay in a mode for hours until it decides to accept new inputs. Currently unable to enter diagnostic mode by holding the two fan speed buttons. This leads me to believe that the programmer is the source of my problems.

Are there any tests I can perform at home to confirm that the programmer is the source of my problems?

Does anyone know of a repair station that I could send programmer unit to for service (diagnostic and rebuild)?

Anyone know of a source for new units (besides the dealership)?
Is the unit unique to the Vette - or is it used in other GM models / years?

How about used units - any sources?

And finally - anyone with experience is changing their unit? After reading the shop manual - I'm still a bit confused as to how to access the unit under the dash.

Appreciate any thoughts -