I'm hoping someone's "been there done that ......"

I'm selling my '92 300e.
Trouble is, it's now developed a no heat on high fan speeds.
There's plenty of heat when it is on low fan but it cools as fan speed increases....checked the usual suspects, like aux' pump, mono valve and coolant pump..
The overhead temp' sensor is clear and okay and the temp' of the coolant is 189f to 212f at the 'stat.

I replaced the coolant pump a while ago for leaking and did a head gasket too for oil leaks...the usual.
to flush to oil from the cooling system I ran TIDE Liquid in the water and filled, ran to warm and dumped it several times with clean water and also ran the garden hose through the coolant hoses and separately through the heater supply hose and there was a good flow with no apparent restriction.

Now I go to test it before the sale, and the weather is cooler, the heater will not put out heat on high....well, just a little warmer than ambient...which right now is a little over 90f at the vent with outside air at 45f.

Anyone had this problem and know a fix before i go and dismantle the dash to check the blend doors? (A/C will freeze you out.)

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