update: Went to my boat this afternoon to check out the glow plugs and here is what I found:
A previous owner did install a glow plug relay so that the glow plug switch on the instrument panel only activates the coil of the relay which is located on the port wall of the engine compartment, see attached photo. Also he rewired the glow plug and starter switches so that they are independent, wish he would have documented these changes. So now the #10 grey/blue stripe wires carries only low current, still goes through the dreaded trailer hitch connector but low current.
The relay is wired so that the green #10 wires goes directly to the starter post where you get 12VDC when you turn on the main battery switch, and this starter post is wired through the main battery switch and then to the starter batter, so lots of current available. Pressing the glow plug switch (with the ignition key on)) puts 12.5VDC across all three glow plugs so no short there, still could be open but no short.
With my DC clamp on ammeter (best instrument ever) I measure 30 A to all three glow plugs. The resistance of each glow plug is reported to be of the order of one ohm so I would expect 10ADC for each plug or 30A total. So it looks like all three plugs are OK.
The engine did start after a 10 sec glow plug heat but took 1-2 sec to catch, so there still may be a problem but I think it is not related to the glow plug switch, relay or the plugs themselves. Before took many tries to get the engine started and a good explanation would have been no glow plug action, so maybe the switch or the relay are intermittent. Certainly others on this site have reported intermittent glow plug switches. I have ordered a new glow plug switch, the present one is probably the original so makes sense to change it out.
Bob, I am amazed that you removed all three glow plugs without removing the intake manifold and maybe even some of the fuel lines, see the attached figure. You can see the glow plug jumper wires just below the black wires and even make out the nut on the front glow plug just behind the center fuel line. I can probably twist off the nut holding the jumper wires with a long nosed vice grip, getting a screw driver blade in there with the intake manifold in place looks impossible. But how did you get a 12 mm open end wrench on the glow plug itself and still be able to turn it. Maybe on the rear one but the front and middle plugs look like I could get a wrench on it but no room to turn. Did you make a special wrench, like a "crow's foot" or something?
Christian, you said you removed all the plugs on your M25, the photo is of my M25XP, is there really more room on your engine??
For anyone who has removed the intake manifold, were you able to get at all the bolts, and did you need PB Blaster of Kroil and if so where exactly did you squirt it? Squiring the bolt head does little and squirting the base of the intake manifold looks hopeless because of the gasket there?