Make sure the foam hose from your in-car temp sensor has not flaked apart. It is about a foot in length, connects to the plastic tube on the sensor, then to a hose coming off the blower motor. The blower motor creates a suction, drawing inside air over the temp sensor.
I usually use a piece of copper pipe insulation reinforced with black electrical tape. It's available at hardware stores, 1/2" diameter, comes in three foot lengths. Alan