Troubleshooting broken speedometer

[ggehrke at gmail.com (GGehrke)]

My speedometer stopped working suddenly last summer while working on
the car.  It worked on a short drive the other day but then quit
again.  I've looked at the line that connects transmission>EGR
thing>Speedo and it doesn't have any glaring problems.  I also
unscrewed the fitting where it joins the speedo and retightened it
(because I thought pushing and pulling on the dash to work on wiring
may have disconnected it or something).  I'm quite frankly afraid to
pull my dash apart any more right now because I'm afraid it will mess
up my other really finicky wiring.

What should I do to troubleshoot this one?  Are there common specific
parts that fail?  The bentley (and haynes and clymer) manual has been
surprisingly unhelpful on this one.

And it's an '80 S dash in a 79, if that matters.  Thanks.

-Grant-