"Funny" you should have this issue-we did-we also have an '89 wagon. Yesterday my mechanic came over, fixed it in our driveway. He had a “new to me” harness which he used to replace the old one. He did pull down the headliner at the back corner and took off the things that cover the inside of the hatch. then he unbolted the hinge (we also used a broom to prop up that side of the hatch) from the bottom and there is another screw you have to get off from inside . look up in the hole you see a philips head bolt . Once that was out the hinge was loose. He then proceeded to pull through the old wires from the hatch-when it was showing at that hole he attached the "new" wires to the old and pulled it back on through. Stripped and attached the wires. easy. He did this at both ends. pushed back in the wires, replaced the headliner and-everything and voila-wipers work. Of course we tested it before we replaced the headliner etc. Be careful with the rubber gasket as when u begin to tighten down the hinge it can move around.
Forgot to mention-he did of course have to cut the old wiring. Then pulled it through-both ways-then spliced the "new" ones together