i have a 1989 F250 7.3 deisel and the glow plugs dont work. i have checked the plugs with a test light hooked to the + battery post and each lit up.someone bypased the ignition switch with a push button to start the truck and when i turn the key on the relay clicks and after 15 seconds it clicks again but no power gets to the glow plugs. however when i push the button juice gets to the glow plugs and after 15 or so seconds it will start .im thinkin that when they bypassed the switch they left overlooked some thingthats gotta be a strain on the starter any suggestions