'94 Ford F150 starting problem ... HELP !!?

I have a 1994 Ford F150 5.8L V8 and no matter what I do I have the same problem. When I try to start the engine the starter continues to turn over even if I turn off the ignition and remove the key but the engine does NOT start. I have replaced the battery, starter (including solenoid), relay on the firewall & ignition switch with NO change (all brand new parts) and still the same problem. The only way to stop the starter from continuing to turn over is to disconnect the battery cable. The truck has been sitting for about 2+ years if that helps.