Check the hinges where the cables that feed the tailgate electrics. I am almost sure you will find broken wires. I had the exact same thing on my 1990 245. Replaced left and right cables, and everything worked well.
I got mine from Tasca Volvo.

Used a couple of step ladders, as in TWO to form a shelf using a 2x4 for the tailgate to SAFELY rest on. I did one hinge at a time. BE VERY CAREFUL, THE TAILGATE IS HEAVY AND BAD THINGS CAN HAPPEN IF YOU LOSE CONTROL OF IT. Removed the headliner at the rear corners. Before you start note how the harnesses are routed through the hinges and down the sides of the tailgate, this can save you some frustration, don't ask me how I know.

Now, I am lazy so I removed the individual wires from from their plastic housings on the to be installed harness,which end I don't remember, whatever was easiest. I cut the same end off the old harness taped the new harness with the old one and pulled it through. I did this for both sides. the other option is pulling the old one out and fishing the new one through, I have heard it was kind of a hassle but doable.

The hinges have a gasket for weatherproofing, I added some extra in the form of some 3M windshield sealant from Advance Auto around and the perimeter, feed through holes under the hinges and hinge mounting bolts. It really isn't a bad job, just be careful with securing the tailgate.

Good Luck