Here is the way I did it.
1> Unplugged fan relay. Removed its cover and drilled a hole in the top.
2>I inserted a rubber grommet in this hole and fed four wires through.
3> I soldered these four wires to the four connections in the relay which
are +12v, ground, relay control and relay output. Replace cover.
4>I used a momentary contact push button with a light in the button. I
intended for the light to show me when the fan came on normally.
5> I connected the light to the ground and relay output wires.
6> The switch contacts went to the other two wires; +12v and relay
control.
7> I mounted the switch in the blank spot you mentioned and plugged in
the relay.
Now for test drive. When I hit 40mph I noticed an unexpected result; the
light was on! I stopped at a red light and it went off! It dawned on me
that the fan motor was working like a wind powered DC generator.
In the last two years I have not really needed the extra cooling but its
nice to have. The fan powered light lets me know the motor can free-wheel
i.e. not froze up. This is useful since in the five years I have owned
the vehicle I have only heard the fan come on twice.
Drive Safely & Good Luck
Ken Lewis     <Kernersville NC, The Heart of the Triad>
86 VW Transporter Three Door Pickup
60 Porsche T-5 Coupe
85 VW GL Vanagon<Elvis>lives again!

On Fri, 24 Oct 1997 10:09:12 -0500 Dan Belick <dbelick@PEAPOD.COM>
writes:
>Has anyone wired an auxiliary on switch for the radiator fan?  I'd
>like to be able to be proactive about the overheat situation by
>turning on the fan.  This would be used when stuck in traffic, using
>a drive thru window, etc.  Can you tell me how to wire it in, what
>sort of switch to use, etc.?  I thought I'd use an extra rocker
>switch like the one the rear window defroster uses.
>
>thanks in advance for any help,
>
>-Dan
>84 Adventurewagon 1.9
>