A slow or non-cranking engine condition could be caused by high
resistance between the starter motor and battery:

*  Check that the battery is fully charged.
*  Check the ground connections and battery terminals for tightness
   and corrosion.  Clean and tighten as necessary.

If the conditon still exists:
*  Install a new starter cable or larger diameter,
   Part Number  251 971 228G.
*  Install an extra ground strap from the transmission to the body.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Joel's note:
  if you really want to go through all the hassle, run a large diameter
  wire/cable (about 8 or 6 gauge) from the battery-body ground strap
  connection to the starter motor group connection. this gives the
  starter a direct connection to the battery without having to go
  through the welds of the chassis. it WILL improve your cranking. :)
  i did this on my 1980 vanagon, but it's such a lot of work that i
  haven't 'gotten around to it' on my 88. i got the idea from an article
  in VW TRENDS. they also suggested running a similar (but not so large)
  wire to the headlight ground connections, to improve the brightness
  of your headlights.
........................................................................
from Service Bulletins, Model Year 1990
..........................................................................
Technical Bulletin
Group:    20
Number:   20-90 T01