I know on my '01 3.0, the rear climate control is a function of the front temperature settings. So if your rear is only blowing cold air, try increasing the front temps (or at least the passenger) to warmer settings to see if that has any effect. You may have already tried this, but I just know that fixed my problem. My 2003 Expedition has a completely separate climate control that's in the rear cargo area whereas my X5's rear climate works off the front climate control but it is adjustable to allow for coolER or warmER air.