2000 ML430: in summer the AC works fine, blows cold air. In winter the heater blows cold air. Local dealership misdiagnosed problem at first (thought the vehicle was overheating), then estimated almost $5000 to fix the problem, including in its estimate "ac step motor and 2 a pillar panels". This doesn't seem to match the symptoms; the blower works fine, just not getting heat. Except once in a while it will come on, but we cannot control it.
What is the solution? I suspect a baffle is stuck or something, but have no knowledge of where to look or how to get to it.
Any advice would be appreciated.

Thanks in advance,

Mark

PS vehicle is in West Texas, where AC is a must, but heat is necessary only a couple of months out of the year.

 

Could be heater diverter flap is not working. This is behind the dash. Need to remove glove box and you can see some of this stuff. Major pain in the butt.

Also check the auxiliary water pump that's in the engine bay. It's supposed to push warm water into the heater core.