Check the fuse and relay to start. If these are ok, then check the blower switch, the resister, then the blower motor itself.
Check the fuses first and the blower relay.
check fues if not bad ckeck relay if not relay check blower moter by hooking it up direct to a power source.
Check the blower relay, it may be getting stuck..............
check the blower fuse, temperature sensor on the radiator,switch on the dashboard and the blower relay switch.
check for voltage at the blower motor on all settings. if no voltage check the blower motor resistor or relay. if it has a resistor it will be right beside the blower motor, if it has a relay it will be the relay on the firewall under the hood.it will probably have a purple,orange,blue,and black wires.
The fuse. The blower motor speed resistor. The blower motor relay. The blower motor switch. Check for power at the blower motor connection.
Check the fuse and relay first. Then check the blower resister, then the blower motor itself. One of these is probably the cause of the failure.
Swap the relay with a known good one, many of the relays are of the same part number. Keep in mind that the blower motor relay only controls the high speed function. The blower motor speed resistor and the blower motor switch controls the lower speeds.
Check the 30 amp Blower fuse. If not that check the fan relay, if not that check the blower motor directly from a jump on the fan itself.
Where is the blower motor relay at.
Since you're checking the resistor, I would assume you checked the fuse/relay for the blower motor. If not check that. If you have then there would be an open wire between the relay and the resistor.