[BB72CHEVKT]

I had a local shop install a new heater control, blower motor and the heater core back in November. The truck has a new painless wiring harness that I thought the shop (different shop) replaced all the wiring but it appears from my investigation under the dash that all wires may not have been replaced. I have been smelling the plastic from the fan switch melting every time I turn on the heater but I was thinking it was some by the heater core like plastic debris. The hot wire that make the fan come in is getting hot for some reason. Here is a pic of the plug coming from the blower motor that plugs into the back of the fan controller. There is also usually has a black wire plugged in as well but it melted. The yellow wire is also a little burnt were it goes into the connection. Any ideas why it is getting so hot?