Friend of mine has a '98-ish Taurus, and the heater fan only works on high.

How hard is it to get to the resistor on these cars, and how likely am I to run into extra complications (like rusted connectors that need to be replaced) in the job?

98 taurus resistor

Motor Switch Resistor
The A/C blower motor resistor (19A706) assembly operates in the following manner:

The heater blower motor switch resistor (18591) assembly is located on the passenger side of the A/C evaporator housing behind the glove compartment (06010) .
Three resistance elements are mounted on the resistor board to provide four A/C blower motor speeds.
Depending on the heater blower motor switch position, series resistance is added or bypassed in the A/C blower motor circuit to decrease or increase A/C blower motor speed.
An overheating protective device (thermal limiter) prevents heat damage to the A/C evaporator housing.
Overheating of the resistor coils can occur when the system airflow is stopped or reduced as a result of the A/C blower wheel (19834) becoming locked or unable to turn freely.


Heater Blower Motor Switch Resistor

I might take a look at it later today...

Thanks, Vince!