Most power stokes have a fuse not a relay they are in the fuse box under the hood
Where Is The Glow Plug Relay Located On A Ford F 250 Diesel?
Depending on the year and engine model, it is likely located in the "valley" between the valve covers - most 7.3 Navistars are like this, behind the fuel filter and in front of the turbocharger.
Were are the glow plugs located on a ford f250. 2000 year