Constant heat is often the result of a failed Monovalve! I'm not sure of your model but the late 80's cars usually have the Monovalve located somewhere on or behind the engine compartment firewall probably on the passenger side. You can very easily and simply check the condition of the plunger by removing four screws, it is quick and easy to do.

Although the info is for a 126 Diesel car your car probably has the same system maybe in a slightly different place. Take a look!

http://mbdieseldiy.tripod.com/monovalve.htm

Good Luck!