I don't see anything about glow plugs in either the tech manual or the owner's manual for the 2520. However, page 149 of the tech manual outlines the operation of the engine preheater. It says this:
"The preheater is energized (heated) when the key switch is placed in the ON and START positions...
After 15 seconds, the timer turns off and removes the ground path from the 121 Blu/Grn wires and the preheat relay coil. This causes the preheat relay to open and stop supplying currrent to the 33 and 32 Blk/Red wire and the manifold heater...
When the key switch is in the START position, current is provided from the C2terminal of the key switch to the 31 and 32 Blk/Red wire and to the manifold heater...
When the engine starts, and the key switch is returned to the ON position, power to the C2 terminal and 31 and 32 Blk/Red wires in removed."
So, no glow plugs on the 2520, but yes it has a manifold air intake heater. The heater comes on whenever the ON or START positions are selected on the starter switch, and in either case, it shuts off automatially after 15 seconds.