Where is the location of voltage regulator on a 1988 ford van E250 4.9L?

The voltage regulator on a 1988 Ford E250 van with a 4.9L engine is integrated into the alternator. Unlike older vehicles where it was a separate component, Ford, and many other manufacturers, moved to this integrated design by that time. You won't find a separate voltage regulator unit. If you're having voltage issues, the alternator itself is the likely culprit.