1990 Dodge Caravan

Have you tried flushing the heater core
check if both heater hoses hot if not it may need a flush
Engine cool
lower the coolant level
Locate the two hoses running to the heater core
disconnect them at the engine side leaving hoses connected to heater core throw the firewall
use garden water hose and compressed air if possible
run the water and follow by air throw one side and out the other and vice versa
do that two to three times watching what leaving the heater core you are looking for the good flow and clear water to run throw
also check if you have heater valve and if its working and if its vacuum hooked too
good luck
let me know