Depends on your comfort level with upholstery and hog-rings. If you
don't know what hog-rings are - you might consider letting someplace
that does tackle it.

You have a bad sensor in the butt cushion. Not a big deal to remove the
cushion (a few screws under the seat and some push-pin thingies into the
seat pan).. and disconnect any wiring going to it.

What you do next is up to you - you can disassemble it yourself (which
means cutting a lot of hog-rings, some of which are difficult to get to)
- or take it and the new sensor pad to an upholstery shop who will
probably charge you a nominal fee to replace it. To reassemble it you
either need new hog-rings and the special hog-ring pliers, or - as I
did, some decent tie-wraps which can be used in place of the hog-rings.

I don't have the E36 Bentley manual any more - so I can't tell you how
well it covers this project.. but I'm sure it at least covers seat
removal (which you'll have to do..) so it should get you started. You
can then track down the parts and where most of the screws go by looking
at parts diagrams on www.realoem.com