Where is the differential refill plug on a 1987 camaro?

The 1987 Camaro doesn't have a "differential refill plug" in the traditional sense. Instead, the differential is filled through the top of the differential cover. There's a large cover bolted to the top of the differential housing. You'll remove this cover to access the differential gear and then add the gear oil through the opening. There isn't a separate smaller plug specifically for refilling. There will be a drain plug, though, typically on the bottom of the differential housing.