I have a 2004 endeavor and I have heat only on the passenger side. The drivers side is cold.
The blend door works, It does change from Defrost, Center, Floor. Also the temp **** works because the passenger side will get cold on the cold setting.

As far as the heat only on one side it does this in all modes.
Defrost drivers side cold: pass hot
center drivers side cold: pass hot/burn you hand hot!
floor drivers side cold: pass hot
Rear floor ducts drivers side cold*** hot

This thing acts like i have dual zone and i dont. I really dont want to remove dash and heater box. I did flush out the heater core to be sure it wasnt clogged. the pass. will roast and the driver will freeze. If i push the recirculate button the temp on the drivers side will improve but still a temp difference.

 

For those still looking for a solution, I had the exact same issue on my 2002 montero limited 3.5. The source of the problem.. Partially clogged heat core. The air flows across sections of the heat core to every vent. In my case I only had mild heat on the drivers side left vent, cold everywhere else.

The solution: disconnect both heater hoses where they run to the firewall beside your brake booster / behind the intake manifold. Space is tight in there. The top hose is where hot coolant goes into the core, the bottom is where the coolant comes back out into the engine.

Buy yourself some similarly sized (diameter) clear hose from home depot or something. Two 6 foot lengths are ideal. By the way a bit of coolant will run out on the ground make sure you catch it. I barely lost a quart. Obviously don't do this when your engine is hot.

Now hook up your sections of clear hose to the outlets at the firewall, both top and bottom. Go get your garden hose. Now the key is you want to back-flush the core.. So you'll be sending water from your garden hose up the OUTLET (lower connection) at the firewall. You will see a whole bunch of rust and other gunk shoot out the other hose (catch this in a bucket etc). Soon you will see the water start running clear. Don't be fooled. Let it sit for a while then flush it again.. More and more gunk will keep coming out as the core gets flushed bit by bit. Mine was completely clear after about 15 minutes of doing this. Be sure to stop for a few minutes each time to let the core soak and all the gunk to free up, then blast it again.

Now reconnect your heater hoses, top up your coolant in radiator and be sure to burp the air from the system with the air bleed screws on top of your thermostat housing. 45 total work.

100% problem solved I now have boiling heat again out of all four vents. Best of all - nearly free.

Hope this helps you guys!