96 W210 climate control problem
 

I think the head unit is dead. Display lights up. shows LO. I only have fan speed control, from off to hi. Nothing else works, press any button and nothing changes.

I had similar problem with my 98 E320. Problem was blower motor/regulator.
I had to replace both regulator and motor. In some cases you may just need to replace the regulator.

If the regulator is causing the trouble, disconnecting it should fix that problem, but you'll have no fan.

I wouldn't think that a bad regulator would cause the head unit to go nuts, but I will easily concede the possibility if it is back-feeding current to the control line.

If it is the regulator and the blower motor is good, you can retrofit a regulator from a W140. The original regulator for your car is NLA, and you need a new motor and mounting plate to use the new regulator.

Right Matt. i had to replace both reg and motor for my car.