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My gut tells me it is the blower motor resistor but you need to confirm power for the circuit at the chassis fuse box, there should be a fuse for that dash heater, on the older Monaco's the lower cabinet that sits on the floor in the center of the cockpit came out and some fuses were back in that area.
the blower motor resistor will be located under the front cap on the plastic heater control box. It will have two Phillips screws located on it. The resistor can be checked, and many times you can see where the break is on them. Using a meter set to continuity will allow you to check. IF I remember correctly, that resistor needs to complete the circuit, so you can turn everything on and probe for power on the wires going into the resistor...(it will have a plug type harness going to)... Hope this helps... Below is a pic of the resistor. (That might not be the exact one but it will look like it)
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