The heater and the AC are two completely separate units. They just use the same vents and fan blower. First check the thermostat to see if it is opening correctly. Does your temperature gage in the dash indicate that the engine temperature is "heating up"? If not it is probably the thermostat. You can also check the thermostat by placing it is a pot of boiling water and watching to see if it opens up. That is a good place to start.....
Check the heater hoses going into the firewall. If both are hot, this is a good indication of coolant flowing and heat being delivered to the HVAC system. For the GC, air flows through the evaporator core at all times and all or some of the air can be diverted through the heater core for heat. This function is controlled by the blend doors and it is a common fail mode on the GC.
The 96 model is a crossover model between the old and new system deployed in the GC. You will need to determine which system you have and look into how to repair it. Diagnostic information and pictures are available on the HeaterTreater GC Ebay listing. Check the listings for both the early model and later model to determine which fix will work for your GC.