what causes the Ac blower to stop working? It comes on sometimes for 30 seconds.

most likely the blower motor is buggered. In Australia its under the dash on left hand side, but you guys drive on the wrong side of the road so it might be over the other side, dunno. On right hand drive model its under the dash on LHS, and takes about half an hour to change. All comes out without disembowelling car. Common issue, but they aren't cheap, especially if yours is like mine and has automatic aircon. Can get the parts off the net. They can't be repaired, and I tried a couple of second hand units before I bit the bullet and shelled out for a new one. For what its worth, I bought mine out of the U.S, cos it was about a third of the price compared to Oz.

I had the exact problem last summer, 2011, with my 1991 300E. There's a specific fuse for the AC blower, which had blown. I don't recall the repair being over $100.