one side or half of seat working.

Classic seat twist. There are many pages dedicated to this that can do far better than I can describe here. Pictures and everything!
http://bellsouthpwp.net/m/i/miales/seatcablesfix.htm
or this:
http://www.nmia.com/~dgnrg/page_11.htm
Go here for all of your maintenance needs:
http://www.e38.org/e34/