Most likely a bad or lose connection at one of the connectors. Probably only have to check the main connector at the resistor & the ones at the mode switch & the speed switch. Pull them & look closely at each terminal for signs of heat or arcing. Before you start take anything apart see if you can get the fan to start & stop by wiggling the connectors & harness (key on). That may help locate the problem.
Here is a link that has some troubleshooting & a wiring diagram for your HVAC. Note that the blower motor speed is controlled from the ground side of the circuit through the resistor, speed switch & the mode switch. In this case it was the motor that was bad but usually it is in the wiring or switches.