1999 jimmy slt fan stopped blowing while in use now can not turn on at all a/c works just not blowing defrost nothing

SOURCE: 1998 GMC Jimmy-Turning Signal Noise

check all bulbs first

SOURCE: 2001 gmc jimmy defroster-heater blower will not direct heat

you have a vacuum leak

SOURCE: fan stopped working in my 1998 gmc jimmy wont blow out any air

Try checking the fuse's, if the fuse's are good it could be a bad blower motor.

SOURCE: 2006 Envoy SLT air suddenly stopped blowing from vents

the resister block needs changing... oddly enough i'm unsure of what fan you are saying is not working.... if it's the front.. the resister block is located on the duct work under the dash for the air... near the fan... you will see a connector going into the side of the duct work. two screws and the block comes out... should be fairly obvious for the rear one but it will be near the fan.

SOURCE: We just bought a 1999 GMC Jimmy and the heater

If you ac is working all the time even when dial switches to hot side try the blend door motor, i had this problem switched motor works fine now