I had a similar problem reguarding the sunroof on my 83 300SD, i would have power everytime i checked it, but the sunroof would only work intermitantley. I traced it to a screw that attaches the posotive wire to the fuse box(it attaches wire to fuse). every time i tapped the fuse box in general it would eaither work or stop working, this is what lead me to this. the only problem with this theory is that on my car the blower is also on the same fuse as the compressor, however if your compressor is a fuse in itself you may want to check the wire connection to the fuse. Hope this helps
Ryan