What will cause the license plate lights to quit working on a 1995 Toyota 4Runner?

the bulbs could be out. if it has a trailer hitch on it and it was installed after market (not by the dealed), a lot of the time the marker lights to the trailer are spliced in to the licese plate lights and the type of conector that are used are prone to corosion. if that is the case just splice it together after the corroded wirer is remove and seal it electrical heat shrink.