Ac/Heat blower motor only works @ high RPM (w126 300sd)
 

For some reason my blower motor quit working, except when my engine is revved up to high rpms. Then it just blows heat, no matter what I do to the settings. Blower was replaced last summer, and the AC compressor is fine (although I wasn't able to be sure if it was coming on or not when i was playing with the settings). I also checked the fuses for the fan, which are fine. I've got a 300 mile trip coming up as soon as it starts to get hot around here, and would like to get this resolved so I don't have heat blasting at my face while my cars going 3800 RPM for 5 hours. Thanks for any suggestions in advance!