Turn on the heater and turn both the passenger and driver side climate temp. to high. Heat should be coming out of both side vents. Then turn the climate temp. down to low on driver or passenger side and check if the temp. changes, then alternate. Both sides should change in temp. as you adjust the climate control knobs.

Or check the link below and read the ''AZC self-diagnostic mode'' section.
http://www.wjjeeps.com/climate.htm

 

or you could google the routine with the AC nobs that pulls the trouble codes from the AC system. It's also in the haynes mannual, I just happen to be at work and not have access to it.

if i remember right press the AC and recir button at the same time and turn the driver side temp knob to the right one click and it will flash numbers. and i think its 53 and 57 is the blend door value being off

 

The guy that just posted before you posted a link on how to test for the codes.