Glow plug light stays on the Mercedes sprinter?

In my experience this indicates a short/open circuit in one or more of the glow plugs. Remove each in turn and test with a multimeter, but be warned, if one has gone, the other will not be far behind.

Had same problem with mine, turned out to be the main red wire going to the glow plug relay had come detached from its connector easy repair 5 mins.Glow plug relay was located just lower down than the battery on the inner wheel arch little black box with wires to and fro Good luck.......