My ac is only coming out of the defrost vents. It happens intermittently. Some days when I drive and turn the ax on it will work correctly with air coming out all the front vents. Other times it will just come out defrost vents. I have checked all the vacuum hoses under the hood from the vacuum ball to the engine manifold and I didn’t find any cracks or loose connections. When the air come out only from defrost vents I notice the climate control switch that selects the vents has not resistance when turning the ****. However when the air comes out the correct vents there is some normal resistance when turning the middle ****.

 

There's definitely a blockage somewhere in your ducting. When this happens, ALL air defaults to the dash defrost vent. This happened on my old '94 Grand Cherokee ZJ, which had BOTH a blockage in the ducting, AND a bad A/C compressor. I didn't end up replacing either, but tracing where there's a blockage in the ducting will likely take disassembling the entire dash.

-Vince

 

Before investigating the mouse nest option:

it seems that the fact that the problem is intermittent and associated with the rotary switch (since you have a 99 xj) feeling funky try hooking a vacuum gauge into the HVAC vacuum system under the dash and see what happens when you operate the switch....both when things work and when they don't.

I'm betting on a bad switch