A small breach in the seal could still allow reverse to engage under certain conditions. The other thing it could be is the reverse clutchs worn. Do a search - there's a number of threads on reverse won't engage and lots of information. The way to avoid San Fran. labor cost, at least partially, is to get a rebuilt unit shipped in. Should be about $1200 from Sun Valley http://www.mercedesdismantlers.com , plus freight plus about six hours to R and R. There's also some information in the archives on repairing just the reverse seals and clutches - it's the first thing you come to when digging into the transmission. Again, the R and R is the big deal.