1999 Ford Windstar Brake light will not work on passenger s

The passenger side brake light will not work when the brake pedal is applied. I have replaced bulbs on both sides and swapped sockets with the other side that is working. If I remove the bulb and then apply the pedal I get 11.8 volts at the pin and I can slide the bulb in the socket and it will light up, but if I release the pedal and reapply, it will not illuminate and I will only have 11.3 volts on the pin coming into the socket. I have found 5 grounds on the van so far and have cleaned them all. Do you have any ideas? Thanks