It might be the blower motor resistor. It happened to me last summer on my Z71 that I have since gotten rid of. The resistor on the Chevy is attached to the blower motor and was relatively cheap to replace (35.00 or something like that). The problem was that it had melted the factory connector which was on backorder at the time due to this problem and cost about 150. I bought the resistor at the dealer and went to a junk yard and cut the connector and about 8 inches of wires behind it. I wired it together with crimp connectors and it worked great.