7.3L Glow Plug Relay, F81Z12B533AC
If you have a bad relay or relay failure this is the first item to replace for hard cold weather starting issues. The glow plug relay sends power to the glow plugs. Common failure part typically needs replacing 1 a year or every other year depending usually on the climate. The Alliant brand relay outlast the generic parts store relays by a large margin. Oem quality Alliant Power replacement part.
*To test your relay, connect your test light across the two little terminals and activate the glow plugs by turning on the ignition. If your test light comes on, the signal to the relay is good. Now connect your test light's clip to engine ground. With the ignition off again, touch each of the big terminals on the relay. One will cause the test light to light - this terminal is connected to battery positive. Now, cycle the glow plugs on again. The opposite big terminal should now be getting power. If not, the relay is bad.