Hi Fishtail,
My sister had the same problem with her Liberty last winter, and as it turns out, her son was helping her clean the car one day and she didn't notice that he pushed the A/C button so the A/C was ON the whole time. This is one thing you can check, make sure that the A/C is turned off. On the newer vehicles the air conditioner is run electronically so you have to push a button to turn it on and push it again to turn it off.
If that's not the problem, try replacing the thermostat. If you're going to replace the thermostat, a system flush is also recommended. You can also check the flow through your heater core as another fellow answered, just put your hands on the rubber hoses that run up to your firewall. If both hoses are hot then your heater core is ok. I have seen people disconnect their heater cores entirely by pulling the hoses off and mating them with a plastic union and putting hose clamps on them so the don't leak...this might also be something to look for. If they have been disconnected and mated like I described that's an easy fix too, but it has been done for a reason, likely a leaking heater core ...just loosen the clamps and take the union out, put the hoses back on the heater core and tighten the clamps and you're good to go. If it leaks then you need a new heater core, which is located behind the dash on the firewall and will require that you take half of the dash and ventilation system apart to replace it.
Good Luck, I hope it's something simple that don't cost you a fortune again!
