1982 300D - heater issue

[simonshearer]

I have searched for a similar set of symptoms without success.

When I leave work each day I have about an 18 mile trip home. I put the heater on mostly using the basic setting without the A/C compressor on, but it doesn't make any difference if I set it with A/C.

It takes a while for the heat to come on (does it rely on engine temp?), but by then I am on an interstate for about 12 miles. It never reaches a really warm temperature (I have it set on 26; outside temps vary between -5 to +5 C at the moment). It is not uncomfortably cold but not anywhere near the heat that comes out later...

12 miles later I slow down for the off ramp (from ~70 to ~30 mph). The heat immediately comes on strong and generally remains good for the next few miles to home - all slow suburban driving.

Any ideas as to why the heat is weak at high speed but good when I slow down? Where is the air intake for the a/c and heat?

Thanks in advance for any help.