At 31 years, if the original hinges, you'll probably want to replace them.

The hinge pin pulls through the front and rear hinge plates. The liftgate sags causing the bottom edge to contact the sill at the bottom.

You can see the 240 lift gate hinge sag at the hinge. With liftgate closed, view edge on from the side. As the hinge plate material wears around the hinge pin, the rear hinge plate (secured to the liftgate) bottom edge will appear lower than the front hinge plate.

The rear hinge sags to the point of the bottom edge about making contact with the roof.

The wires are pulled and eventually break. Through the ground wire appears to fatigue and fail long before that.

The second set of liftgate hinges on the 1990 li'l red 245 1990 wagon (M47 II) were nearly like new when I pulled them in 2006 from a Portland, OR area junkyard. Yet the ground wires in each had broken. I did not replace them.

All electrically works well with that liftgate. I replaced both hinge wire harness a few years ago opting to route the harnesses inside the cargo area.

The lift gate wiper action is slow yet determined. I've lubricated the wiper shaft through the exterior bushing and not yet the interior lift gate wiper arm bushing. The ground connection though the liftgate latch and the hinges appears to be alright. The liftgate DEMIST grid works fine and fast.
--
Kittys are fuzzy and sometimes fluffy! Spokane, WA RULES!

AWS S3 Buckets full of sheeeeeeet.

Yes, I remain in this vermin-filled structure with cracking exterior wall brickwork sheet hole as you see in the image somewhere in rotten Saint Louis, MO:

Repair is possible, doing something like prying the terminals off the rivets, scrunching them a bit with something like vice grips, then re-peaning new terminals, but unless you can source some 14 ga high flex, multi-thin-strand wire, like heavy DVM test lead wire, to withstand the flexing then it won't last long. I think my first attempted repair using regular primary wire didn't even last a year. If the left hinge ground wire is still intact then you don't have to worry about it, you only need one of the ground strap wires. If you're having electrical problems at the tailgate and at least something back there is working then it's likely one of the wire harnesses, extremely common. Sometimes both harnesses have a broken wire by the time you get around to chasing problems, so consider changing both at the same time. Genuine Volvo and cheaper aftermarket harnesses are available, but you get what you pay for. You can figure which side by noting what's not working. BTW List price for the chrome hinges (1315550/1315551) for your 1989 are around $180, but are shown as discontinued, although IPD says they still have them, but at $200, and I see a few currently on eBay for as low as $95 new (left only). The later black ones (3540734/3540735) are cheaper and still available, list price is about $150, currently discounted $122 at Tasca. Used ones will mostly all have pins as worn as yours. As I vaguely recall, there was a minor design change to the hinge around 1985/1986 with an improved ground braid and possibly a minor profile change, but I believe the earlier ones are the same mounting. 140 wagons (145) also used the same hinges in case you see a listing for a used one of those in good shape.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now