Check continuity from ground to - light socket to positive side of socket back to wiring harness. You have either a broken wire or a corroded one whose resistance is high enough to not allow current to flow.
or tap the relay a few times, the one on the right side of the seat. if the turns havn't been used in years the relay is just not used to working right, ride around and the relay should start working properly. speaking from my experiance.
