Ok, I really don't know anything about about the heating system in the e300. The problem that I'm getting is that none of the vents seem to be working. My blower blows strong, my vacuum levels are strong. The only vent that seems to blow anything is the foot vents but with the blower on high they blow very little air. Another thing I noticed is that as soon as I turn the defrost on some air will come out but then it will stop. So if I had to guess there is vacuum but maybe something electrical. I just replaced the ovp switch.