a/c blows hot air out front vents, cold out rear vents. Can i fix this? Not too expensive way?

You might have a stuck blend door in the front. Usually that is due to a blend door actuator failing. That can be a labor-intensive job to replace as it often means loosening or removing the dash. Some car models have easier access to the blend door, and some are nightmares. Google "blend door actuator" for your make/model/year of vehicle to see what they are and how to fix, sometimes people find a shortcut to replace them without ripping out the dash. (disclaimer: your problem may not actually be a blend door actuator, this is just a guess based on your post)

Blowing cold in rear and hot in front

Blowing out cold air in back and heat in front on a 2003 expedition

I have a 2003 expedition. This happened about 4 yeats ago. Ours started blowing hot air out of the dash ALL THE TIME. With air on high...it blew only hot air. I actually was able to change the blend door actuator without removing the heater box. (Not easy but can be done). This dis NOT fix the problem. So took to a shop and the mechanic had to remove the entire dash to get access to the blend door. What he found was where the actuator male nipple went into the heater box...it goes into a cavity on the blend door arm. This male nipple is shaped in a 4 way cross pattern so that when it turns, it also turns the blend door. It appears that over time, this female cavity will crack and split. When this happens, the actuator is turning...but it can't turn the blend door. This means the door stays in whatever position it is in. Well...here is where it gets aggravating. Ford no longer sells this blend door. And no one offers this part...original, aftermarket, nothing. One thing we did find was that the blend doors we COULD find had a metal ring around the outer part of that female nipple. So the mechanic started looking and he found that a 3/4" pec ring for plumbing fit around it. He had to sand it slightly and he put glue on it and pushed the ring on the nipple. He reinstalled everything and 4 years later...it still works fine. One other point. Even though our heater core wasn't leaking...I had him go ahead and change it since he had it out. It was a very involved repair. Like I said, he had to remove the entire dash to get the heater box out. This also included discharging and recharging the ac system. Couldn't believe Ford no longer sold this part. It wasn't like it was on a 1940 ford. This is a 2003 expedition and it also seemed like this was a common problem. Anyway, hope this helps