Blows Hot Air all the time - display blank

[mcollinstn]

1995 S500

Car sometimes takes a mental "time out" where Climate Control display freezes. Blower motor goes to full HI and temp goes to full HEAT. Display controls are nonresponsive. This can last 5 minutes or all day. When it decides to "fix itself" it simply strats working properly.

While it is "on the fritz", if I turn on the parking lights (or headlights), the display will go completely blank (digital temp readout will go blank). When headlights are on, the red "on" LEDs on the hvac display will come on. Still remains unresponsive and hot air blasts out violently.

When it is acting up, I can remove fuse 15 and the blower motor quits. Still passes hot air thru the vents, but it is not force fed. When the fuse is out, if I turn on the headlights (or parking lights) the blower fires up and blasts me again.

The first few times this occurred, I could disconnect the battery overnight and it would correct itself for maybe a day. Now disconnecting the battery doesn't necessarily help.

Am I looking at a component failure? A relay? A sensor? A wiring harness? Demonic possession?

I know the dealer will throw high dollar parts at it till it works. Not really what I want to happen.

Ideas?
(Oh, no way to run diagnostics on the HVAC when it is on the fritz. Everything unresponsive).