Hi there,
As the title says, my windshield wipers will only work when the key is in the on position, but when I start the engine, they stop working. My truck is an 08 Ford f-150 4.6l. Any ideas??

 

Fuse F1 and F113 feed the wiper circuits. Fuse F1 is fed from the ignition switch and energized when the key is in the ACC/RUN position. F113 in hot all the time. I'd start by checking the fuses. If they check out ok, I'd check F1 for power when in the ACC (ON) position. If it has power, start the truck and check it again for power while it's in the run position.