I have a 98 grand cherokee , for some reason when i bought it the blower only works on defrost. Any clues what i can do to fix this problem?

 

Welcome to CF!

Sounds to me like a broken blend door shaft. Not uncommon on any jeep. Do a search on the forum and you should find what you need to fix it.

Replacing the blend door the dealer's way requires removing the entire dashboard. There are kits where you cut a hole in the heater box to get to it and replace the piece with glue and sealer.

 

not the blend door, that's for hot/cold.
it sounds more like the selector.

it could also just be a vacuum problem that controls the selector.
i'm having the same problem in my 97 tj. gotta pull the dash to get the hvac out and see what it actually is. i can't see a vacuum leak on mine.

 

Vacuum.... either the vacuum hose is loose or off at the intake or the vacuum canister has failed either way look for where the vacuum line runs through the firewall usually on the passenger side and start back tracking.