No... You misunderstand what I am asking. I will attach a picture of the gauge arrangement I suggest. The picture was done for an article I wrote that will appear in next months "Import Car" on diesel transmission adjustments. The hose is the white MB hard plastic vac line and I am using a 4 way but a 3-way is what is easiest. (vac line 000 158 14 35, 3-way 117 078 01 45 the line is sold by the meter and is a little over a dollar a meter)
What I am asking is that you insert the gauge into the system (AS IT IS DESIGNED - non-intrusive testing so to speak). I am basically wanting to see if vacuum is in either line as this is the easiest place to test (that I could describe in writing). The switch is usually open (especially in Florida), testing on both sides confirms it. If vacuum is here then you usually will have it at the master switch, which is much less convenient to test. The point to all this is your main symptom: only defrost works. This almost has to indicate a problem in the master switch as it is hard to imagine any other blower problem that could cause that symptom.
The switch and all elements will work at very low vacuum levels maybe as low as 3in vac. The system also has a programmed leak so it won't maintain vacuum without the engine running. This programmed leak will throw you. It is a bundle of green wire plugged into a tee. Evidently the vacuum makes it out through the strands of copper.
The system in the SEL is basically the same. There is one different switchover valve and some different vac elements as the dashes are different but the designed and control concepts work in either the same.
Bosch Master, ASE Master, L1
33 years MB technician