Todd,
  Here is how I did it, and it had an unexpected benefit. I used a momentary
contact push button with an LED built in. Very similar to the player buttons
used on coin operated Atari arcade games of the 80s.  ;-/  If I was going to
do it over I would use a toggle switch. I removed the relay that controls
the radiator fan's high speed. To which I removed the metal can top and
soldered a wire to each of the four terminals inside. The metal can got a
hole drilled into the top to accept said wires before being reassembled.
   I am going from memory so this part is vague. The LED was attached to the
wires that went to ground and the plus terminal of the fan. I am sure there
was an appropriate resistor in series. The switch contacts went to the other
two wires, +12 volts and the relay coil's control line.
   As expected, the LED lit brightly when the fan was energized and  mashing
the button forced the fan on. The unexpected result was; as you started to
drive, air flowing through the radiator made the fan's propeller rotate.
This caused the motor to act as a generator and produce voltage, in turn
illuminating the LED to about half brightness.
   Benefit? As long as the LED glows at any speed above 15mph you know the
fan's bearings have not bound up! QED.
YMMV,
Ken Lewis
http://neksiwel.20m.com/
Vanagon mods, tech info and O2 sensor help