I did this on my 81 Jetta a number of years ago.  I don't remember the
specifics, but it was basically swap out the unit (much easier on the van!)
replug and run a wire to the coil (I did it by trial and error!).  This part
was a little easier on the jetta with the engine in front, but shouldn't be
too hard.  I'm sure someone out there can be a little more technical about
this, but it is relatively easy.  You also have to decide if you are just
going to live with the mileage on the new spedo/odometer or if you want to
swap with your old one.  The old one will fit, but I believe it is a
slightly different shape so there are some gaps around the trim.

Cory Sammons
84 Westy with PLEASE HELP starting problem!