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.