My car's a 70 so it might be a little different. Open hood, and slide out side marker fixture (depress a little plastic tab, might need a screwdriver). If you have headlight wipers remove them, too. Unscrew the little bolts (three of them) and the fixture comes right out.

To remove the lens, just snap off the little retaining clips and the lens comes right off. R&R for each side is 10 min max. The new lens will come with a new weather seal and clips. To really upgrade the lighting, get e-codes for both sides if you don't already have them.

As to the bad ground, easiest repair is to run a new ground using those Radio Shack wire taps that clamp onto the wires. The ground wires in Volvos are black.
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David
98 S70 T5SE // Black, misc mods (mostly lighting), red calipers
92 940GLE // Hella Micro DE foglights