Horn relay is the first thing I thought of when seeing it but have not done any research or looked in the EVTM. Should be able to find it in that someplace.
Eddie,
81 F100 Custom SWB, 5.0L, 4x2, Single Rail Four-Speed Overdrive, 3.00 Non-Limited Slip Rear Axle, Non A/C truck to complete 85 F150 factory A/C conversion, Nutmeg interior color.