the grid heater should be coming on, if the air intake temp sensor read temps below 60 degrees, pcm then opens circuit to heater relays and then turn grid heater should come on for 10 to 20 secs on .
is your wait to start light coming on with ignition switch to on position , for how long is it staying on , if any , can you let me know ?
check the fuses feeding power to the grid heater relays
check the grid heater wires i seen them get rotted
you can try jumping the grid heater relay to see if the grid heater will come on , in case you have a bad intake temp sensor that is not showing the computer it reached below 6O degrees to kick the relays in .
1994 - 2002 Ram Manifold Heater Relay Location
The 2 intake manifold air heater relays are located in the engine
compartment, attached by a single bracket to the left inner
fender below the driver's side battery.
1 - battery & Tray - Driver's side
2 - relay Mounting Molts (3)
3 - relay Control Wires (4: 2 From ground and 2 from ECM)
4 - intake Manifold Heater Relays (2 on a single assembly)
5 - rubber Wire & Terminal Insulators (4)
6 - feed Cables From Battery Terminal (+12V)
there could be a problem with fuel injection pump , it need to be tested , but that requires special fittings and diagnostic scanner i suggest you do that at dealership or diesel repair shop .
let me know if u need anything else