Loren, the issue is not that the early switch won't SEEM to work ok in a
very late van. The problem is that if you use it in a van wired for the
later switch the early switch will bypass the protection fuse for the
dimmable dash lighting. VW changed the dash lighting wiring when they
went to the later switch and the fuse is in a more logical place in the
later setup. The dash lighting wiring is a common cause of melted dash
wiring in very early vans with no fuse, 80-84. VW added fuse protection
beginning in 85 models. Because the dash lighting wiring runs all around
to various things in the dash area it is important to have it protected
by a fuse. Using an earlier switch in a 90/91 meant to have the later
switch has the effect of removing the fuse protection.

I don't know exactly when VW implemented the 90/91 switch. My opinion is
that the headlight switch was changed to allow better integration with
the DRL option. Daytime Running Lights that is. VW needed to isolate the
dimmable dash lighting so it could be always on with the DRL even with
the main headlight switch off. Before that the dimmer wheel got power
from the main part of the switch only when the switch was on. With the
later switch setup and the DRL option installed the dimmer wheel works
even with the switch off.

Mark

Loren Busch wrote:
> ​RE: Headlight Switch
> Ken, not trying to challenge a guy who stocks and sells this kind of stuff
> for a living, but are you sure that it's both '90 and '91 and not just '91
> that has the extra pins?
> Second, when the headlight switch on my '90 went out a few years ago I
> replaced it with an older switch out of the used parts bin at my local VW
> shop and it's been working fine for many years now (relays added).
> And third, do you know the part numbers for the different switches?  At
> that time we looked at the part number on several headlight switches and
> found several letter suffixes on different switches.  Two different
> switches with different numbers (and IIRC different pins) seemed to work
> just fine in my '90.  I put in one with the same suffix that had been in
> the one that went bad.​
>