how do i fix my saab 900?

This could be a fiddly job, and I can't tell you exactly how to do it, but I think you need to check the earth to the dial/gauge.
My Grandad had a similar problem with a Daewoo (fuel guage always read empty) - turned out a new earth lead fixed it.

The dials may be earthed individually, or there may be a common earth for them. Check the earth points around the engine bay (maybe under the dash too?). A contact cleaner will ensure a good electrical connection. A dab of vaseline on the top will stop further corrosion.

Hope this helps!

Mr Smith
November 2008