99E300TD - Lack of cabin heat

[BootieNH]

I scanned some historical posts about the topic of lack of heat in the cabin but thought I'd post anyways... In general, the climate control works ok in my car, but I have some quibbles. During sunny daylight hours, the climate control always seems to overcompensate towards cold. I can set temp control to 75 and it will still be blowing cold A/C. Leave the control at that temp, and by nightfall, the car will magically start to pump lots of heat into the cabin - subjectively the car will get much warmer for a given temp setting at night than during the day. Also, during a long winter drive, I noticed that no matter what temp setting I used, I could never get the heat to an adequate level in the cabin. Only when I set the control to DEFROST (both sides on HI) did the heat come on - in spades. It could cook the hair off my head. So I kept flipping between DEFROST mode and OFF to just try to keep the cabin comfortable. I'm familiar with how to access the diagnostic mode of the climate control, any suggs on what I should look for? thx