Re: 2005 freestar
You may have a partially blocked heater core. If you haven't changed and flushed the coolant since the vehicle was purchased (if you're the second owner then you should change all fluids anyway for better piece of mind!), it'd be a good idea to do so. The blend door may be sticking, but there isn't a blend door for each vent tube. It's an assembly located under the dash that is vacuum actuated by the control knobs on the dash. It's also not an easy job to do, even for yourself, but you'll save a weeks worth of pay if you do the job yourself. I would do a coolant flush just as it's the cheaper possible fix.
Pressures can vary between vents due to bends in the tubing as well as diameter. Look under the cowl (below windshield wipers) and make sure there is no debris blocking the intake area for the fan. You may have to open the hood.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!