Easiest way to check those Glow Plugs: Disconnect each
glow plug. Connect a test light to the POSITIVE side of
the battery and touch each Glow Plug. The light should
shine. No light - open Glow Plug. Once any Glow Plug
concerns have been addressed: Cold - key off - turn on the
dome light and watch for it dimming slightly when you turn
the key on (not cranking the engine - just on). You can
hear the relay click and see the light go bright and dim as
the controller cycles waiting for the engine to start. NOTE:
if you are experiencing temperatures lower than 40 degrees
overnight, consider plugging in the Block Heater. If the
Glow Plugs and relay/timer are ok - I'd look at air entering
the fuel system but that's a tougher diag.