The neutral safety switch on a 1978 Ford F250 is located
on the top of the transmission. It's a small switch, often cylindrical or rectangular in shape, connected to the transmission's shift linkage. Precisely where on *top* depends on the specific transmission (e.g., C6, Toploader, etc.) but it will always be physically connected to the mechanism that indicates the transmission's position. You'll likely need to get under the truck to see it clearly.
Because of the age of the vehicle and potential variations, a detailed picture of your specific transmission would help pinpoint its exact location. A search online for "[Your Transmission Type] Neutral Safety Switch Location" will likely give you a more visual reference.
