These two previous tips are right on. Be sure to check that switch located, (as you face the rear of vehicle) it will be on the drivers side close to the alignment pin receiver. This switch does not need to be functional and bypassing it may indeed correct your problem. If the switch has gone bad it will result in an open circuit rendering the keyed tailgate switch and dash switch both inoperable. If connecting the two leads together doesn't restore operation of both switches, I advise checking the keyed tailgate switch itself. The stock component is comprised of phenolic plastic (same type of plastic used in distributor caps and other electrical applications as it is non-conductive.) This type of plastic is also fairly brittle and age contributes to this. As well the pole positioning lever located inside the switch body is plastic as well, although not phenolic. Obviously the fatigue of movement and stress on these will lead to breakdown. I know the replacement part is available from the dealer and isn't all that expensive (around $40.00). Well worth it as a wagon is greatly limited without access to the cargo area. In closing I should mention I'm the proud owner of a '91 Grand Wagoneer and she is near and dear to my heart. I'm pleased to see all these testimonials and feel the obvious warmth and compassion we all share for this truly distinguished American icon.
'89 GW, everything but the moon roof (has the luggage rack). Front end damage into radiator, but rendered it re-usable by using a tow strap and a covered parking post! The rear window quit, was getting the switched power, but a relay took care of it.
A starter, battery, fuel pump, alternator, brake indicator that fell off and ground the bracket for a month (the disc still held up), the water pump, fuel filter... lots of headaches. But the looks and comments are awesome. As well as a switch for 4WD!
900w P-P (420w RMS) stereo with room for 8 speakers bugs everybody around me! No heater/AC... :-)!! Cracking seats also, but overall, a monster with Quadra-Trac!!!