Pull your heater hoses off of the engine and back flush the heater core with low pressure from a water hose. NO MORE THAN 15 psi !!!!!!!
Coolant does not circulate through the heater core for several months and sludge cloggs the core.

If you have the Auto temp control; and you have checked the coolant level then you may need to replace the module that is under the dashboard on the right side footwell,. this is a common problem on these cars. When you remove the plastic under dash panel, look up and there is a module in the duct for the heat that determines the temp so the Climate control works correctly. I pulled mine out of a junkyard car and saved about 80 bucks.