Often times, the problem may lie in changing the Heater Blower Motor Switch Resistor, located on the heater plenum and is connected by a group of wires. This simple device can get messed up with dust, oil and dirt after a while and disables the heater. If it's not the wires, I bet you that its the resistor.
Look on
CJ Pony They aren't much and don't take all too long to replace. Just make sure you have superglue.