I'll do my best to explain the symptoms, and then look for a recommendation on which sensor is the likely culprit. Thx!

1999 E300 TurboDiesel climate control (AUTO mode)
Summer months - No lack of air conditioning. Car gets plenty cold. During daylight hours, car pumps out A/C seemingly regardless of temp setting. Set to 76 and car will cool much below that. Soon as the sun goes down, or if it's a dark overcast day (or outside temp drops??), car seems to respond "properly" to temp setting, actually pumping heat.
Winter months - Lack of heat. Cabin does not seem to warm properly, regardless of temp setting. Setting to DEFROST mode (HI) though, and it gets hot enough to blowdry your hair. Lack of heat more noticeable on extended highway trips.
I know how to access the diagnostic mode of the climate control, can anybody make any recommendations on troubleshooting this?