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.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now