You are on the right track for the resistor. It is in the top right hand portion of the HVAC housing in the engine compartment. You can test it with an Ohm meter. Even a continuity test will tell you if it is working.
The blower motor relay hangs from the firewall just above it. The high speed setting should be controlled by the relay.
Unfortunately I believe the control switch has been a problem in the past. It may be your next suspect.