My tail lights, brake lights and turn signals on the tow vehicle work. I attached a 4 way trailer lighter tester to the tow vehicles harness. As stated the tail lights, running light, brake lights, and left turn signal are operable. The right turn signal is inoperable.
1) Verify the tester, make sure the problem isn't there.
2) Check the trailer turn-signal-fuse in the power distribution box under the hood. The trailer light circuits are protected separate from the van's lights.