I have a '99 F150, 4.6... When I press the overdrive switch on the shifter, it doesn't turn the overdrive off. I have checked the switch and it is o.k. I also checked the fuse to the switch, it is also o.k. Does anyone know if there is an external relay for the overdrive? Or, is it inside the transmission?

 

No i'd say you either have a bad switch or a pinched harness.
Alan

 

I doubt that a relay is your problem, but you do have a box with I think 5 large relays for the transmission. I think two of them are for overdrive. The relay box is located under the hood usually on the drivers wheel well. They can be tested for continuity and are costly to replace ($15 each)