Hi, I have a 1987 760 wagon here in UK with euro headlights, as others have said one H4 bulb each side and glass lenses. If you have euro lights then for starters you can try the following easy and cheap fix. Take out each headlight assembly and dismantle the glass lens, making sure to retain the clips and rubber seal carefully. You may need to renew the clips or seals if corroded or perished. If the glass lens is badly chipped, you can replace it from volvo (expensive) or other source. If the reflector is bad, you can replace the whole assembly. If parts are not too bad, proceed to clean the lens in the kitchen sink with jif or similar and a cleaning pad with a scouring surface. Clean the glass well both sides until sparkling. Then take the reflectors and carefully clean with a soft lint free cloth. On mine the metal holding the bulb had corroded a bit over time spreading a fine layer of dust over the reflectors which dimmed the lights. Reassemble the headlights and reinstall in car. By the way the Haynes manual is OK for this work which is pretty easy. Finally, after reinstallation, you can decide with your normal H4 bulbs whether the light strenghth is acceptable. If not, the easy fix is a special H4 bulb from Philips, Osram etc. which throws out 50% more light. Ding these simple fixes on my car dramatically improved visible light output. If you want more work and better and permanent effect, go with John Sergeant's suggestion for a relay and voltage upgrade.
Best of luck.

Kjartan Langskog
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Kjartan Langskog