Could use some help here...

Would appreciate anyone with an 88 or later model syncro or non-syncro
(preferably a 90) looking in their engine compartment and telling me what
size wires they have from the alternator to the starter....

Or if someone has resource that could give the wire size with the
appropriate part number for a 90 Westy Syncro

[if you can't actually measure the wire gauge... you could compare to a
scrap of U.S. wire... 6mm should be very close to #10 AWG size... and 10 mm
would be larger than a #8 AWG size... OR perhaps you could measure the
diameter of the outside of the wire insulation with calipers]

here's the story:

Mechanic replace the wiring harness that goes from the alternator to the
starter in my 90 Westy Syncro...

Basically the wire sizes look to be:
2 wires 2.5mm from alternator to post in junction box at left front of
engine compartment...
1 wire 6.0mm from the same post in junction box to starter....
all 3 wires packaged in a Y shaped harness

BUT...  BENTLEY page 97.204 shows a single 2.5mm wire from alternator to the
junction box and a 10mm wire straight from the alternator to the starter for
a '90.

I called the local VW dealer and asked for the appropriate harness... they
looked it up... same part number (251 971 349 M) as that listed on my
mechanic's invoice... apparently the parts manuals do not show the wire
sizes.... they just call up with the part number and assume that what is
delivered is the right part... Could they have sent the wrong wire
harness???

Bentley shows that not since '86 was there a 6mm wire used to the starter
and a double 2.5mm to the junction box... Since '88 they show a 10mm going
straight from the alternator to the starter.

Thanks for your help... much appreciated... I need real world facts to go to
my mechanic with...

Bill