Caveat - I have only done this on my car; a 1992 diesel jetta CDN spec.
But the intermittent relay may well be a common part. Mine is marked as follows: a VW and an Audi logo;
191 955 531 UH2 12V SIEMENS 72 WV 006 MADE IN AUSTRIAIt's not that hard, really.
You have to a) remove the intermittent relay from the car b) remove the cover from the relay c) find the solder pads for the electrolytic capacitor nearest the relay base d) bridge this capacitor with both a variable resistor ("pot") and a fixed one, in series e) make a tidy installation on the relay case and on the dash
details for a) the Bentley book says the intermittent relay is in the following places:
1985 to January 1989 (VIN 1G-KW400 000) Jetta - 3rd from right on bottom row (#10) January 1989 (VIN 1G-KW400 001) through 1992 Jetta - 2nd from left on bottom row (#8) 1985 Golf and GTI - leftmost in upper row (#1) 1986 though 1989 Golf and GTI (US production) - leftmost in upper row (#1) 1989 Golf and GTI (Mexican Production) - - 3rd from right on bottom row (#10) 1990 through 1992 Golf and GTI - - 2nd from left on top row (#5) or 2nd from left on bottom row (#8)You can determine your year and place of production from the VIN. Letters in the 10th place determine model year (FGHJKLMN where F=1985, G=1986, etcetera) Letter in the 11th place shows place of manufacture (V/W/M meaning US/GER/MEX)
details for d): I used a 2.2K fixed resistor and a 100K variable one - at minimum delay you don't want the cap to drain too quickly, so 2.2K is a reasonable low limit; and 102K is pretty close to an open circuit, so the delay at maximum is almost as much as the normal delay. A 10K pot did not give quite enough maximum delay, and I suspect a 1meg one would have most of the useful range of delay in a small part of the sweep. Try to get a linear taper pot - and audio one will work, but the response won't be, um, linear.
details for e) I soldered a pair of wires, one to each pad and brought them out of a hole in the relay case. I put a dollop of hot glue to seal them at the hole, and then terminated them with insulated spade terminals. I mounted the pot in a blank switch plate (make sure you get a pot that is physically small enough to fit through the hole when you put the plate back in the dash). The pot wires ran down beside the instrument cluster and were spade-connected to the relay.
If for some reason you want even more delay than stock, replace that cap with another larger one. Make sure you use one with at least the voltage rating on the original (25V on mine).