Where is the glow plug relay on a 1999 ford 7.3 diesel?

The glow plug relay on a 1999 Ford 7.3L Power Stroke diesel is located in the Power Distribution Box (PDB), also sometimes called the fuse box. This box is typically found under the hood, on the driver's side. It's a fairly large rectangular box with many fuses and relays inside.

You'll need to consult your owner's manual or a wiring diagram to pinpoint the exact location of the glow plug relay within the PDB. The relay itself is usually identifiable by markings or a diagram on the inside cover of the PDB. It might also be labeled as the "Glow Plug Control Module" or something similar.