_____
Is the E300D glow plug system similar to the 300SD?  If yes, then the
plugs are the fast type and are powered in parallel via a large black
relay.  You can check each plug's resistance individually by pulling the
connector away from the relay and checking inside the terminals.  You
should read something like 0.3 Ohms.  Plugs like this usually fail open,
and resistance will obviously be infinite.  On my 300SD i found two bad
plugs this way.  You should read 12V at each terminal immediately after
the ignition is turned on (connector plug removed).  If not, the fuse
may be bad.  The large fuse (metal strip) is inside the relay just below
the cover.  Also, check all connections in the main supply cable to the
relay for voltage.

You have to remove the INTAKE MANIFOLD to change the glo plugs?

Oh. My. God.

I thought removing the fuel rails was annoying enough even though it only takes
10 minutes:

    http://articles.mbz.org/engine/diesel/plugs/

Well, if you can't watch the glo plug glo can you measure voltage on that
wire someplace?

Although it really seems to me that anything such as a relay is gonna give
you a go/nogo for all the plugs and the only thing that would make individual
cylinders work or not work would be the glo plug itself. I guess in a perfect
world you test them before installing something that impossible to check in situ.