A 1988 Ford F-250 doesn't have an inertia fuel shut-off switch as a standard feature. These switches were more common in later model years and in different vehicle makes and models. Their purpose is to cut off fuel flow in the event of a collision, to reduce the risk of fire.
If you're looking for a fuel cutoff for safety or other reasons in your 1988 F-250, you would need to install an aftermarket one. There are several options available from auto parts stores.
