I just went through that problem on my 88. The thermostat could be bad, use the 195 degree one.
You can also check the heater door just inside the firewall behind the blower. You can remove the glove box and check and see if the cable for the heat is moving the lever for the door.
The other thing to check is to see if the door is slipping on the bar itself. If it is it will not open enough to allow the air to flow. It happened to mine and we had to spot weld the door to the shaft, heater is working alot better now.
Thank a Vet for Your Freedom
1988 K5 Blazer, 350 TBI
35x12.5x15, 9500 Winch
Rapid Recovery Vehicle
Can You See Me Now?