Successfully trouble shot my back-up light issue this weekend.  Below are
the steps taken.

Tested continuity of the switch located on the driver's side of the
transmission.  Remove the leads to the switch, place van in neutral, and put
each end of the meter on the switch spades, the ohm meter should read "open"
or no connectivity.  Now put the van in Reverse, the ohm meter should read
"closed".

Now that you know your switch is fine, test the leads for power.  Put a
circuit tester on each of the two leads, while the Van is in reverse and the
ignition switch in on (do not start the van).  The horn should work, if not
check fuse #12, as that is the same fuse which serves the back-up lights.
Once you find the power lead, the other lead is going to the bulbs in back.
If you put an ohm meter on the lead, and the other end to ground. you should
get a "closed" indication.  If not, check your bulbs.  If you get a closed,
and have power from the other lead, everything is in order and the lights
should work.

My problem was, the lead from fuse #12 was not connected properly to the
switch, and each of the two leads by the trans switch both were wired to the
back-up lights.  I pulled the fuse panel, and removed the lead which was to
go to the trans switch.   There are two spade connectors on fuse #12, if you
pull one and the horn still works, you have the back-up lead in your hand.
I then put a 14 foot long wire onto the back-up lead and brought it to the
rear of the van.  I pulled the wiring box in the engine compartment, and
looked for a "black" wire.  Testing first for connectivity to make sure
there was no power to the lead, I then put an ohm meter across the lead in
the box and the wire ran from the front panel.  Bingo, found the lead to the
front, placed a wire from that to the switch on the tranny, and connected
the lead to the back-up bulbs to the other.  And, back-up lights work again!

Jack R.

84 Westy Wolfie