My girlfriends 98 cav has a problem with the heater fan. It will only turn on if on the highest fan setting. for the ac also. Now i have the repair manual and can go to pic n pull and get another blower unit from under the dash, but is that what it is?

It's likely the resistor pack that is used for lower fan settings. The high fan setting bypasses the resistors.

And where would the resistor pack be? i have the haynes repair manual but could not find it in the section.

I can't say exactly for the 98. My 97 shop manual says:
"The blower moter resistor is located in the lower right corner of the HVAC module, between the blower motor and the front of the dash."

I can't say exactly for the 98. My 97 shop manual says:
"The blower moter resistor is located in the lower right corner of the HVAC module, between the blower motor and the front of the dash."

They're all in the same location from '95 to '02, under the glove box on the passenger side. Remove the plastic panel under the box, remove the blower motor (3 screws) and wire harness, then you'll see the resistor unit. 2 screws hold it in,along with another wire harness, 1 of the screws is very hard to get at. I took the passenger seat out to allow a better work environment, as you need to be a contortionist to get these things out.