How cold is it where you are? Even if the thermostat was stuck open, seems like they should eventually heat up at least enough to provide some warm air. Have you had them checked out. And on the Cherokee at least, have you scrolled through the info screens on dash? You will find the actual coolant water temperature there. In summer mine will run around 200F, but in 10F ambient, hardly ever gets above 175F and that's several miles down the road after a 10 minute warmup at home. Dealer should be able to tell if it is in the coolant temp or if something is wrong with climate control.