Hello, and welcome to the forum.

Normally, for diesel related questions, the "Diesel forum" would be the place to post your question.

Diesels that fail to start are a hot topic right about now. I'll provide answers here for you:

You are correct in that the problem lies in the glow plug circuit. The problem may be one or two failed plugs, or the problem may be in the glow plug relay itself.

If you are somewhat handy with a voltmeter/ohmmeter, you can perform the following tests:

1) See if you have 12V going to any one plug when you turn the key to the preglow position. You must get setup to do this prior to turning the key because you only have 40 seconds or so. The plugs are a bit difficult to get to on the 603, so select the easiest one to attach a small test lead. If you do not have 12V at the plugs, report back for further advice.

2) If you do have 12V at any one plug, you then need to disconnect the cable at the side of the glow plug relay and perform a test to see which
plug(s) are NG. The glow plug relay is a black box that is on the forward side of the driver's fenderwell. There is a cover that will slide off the top of the box. The box is about 2" x 3 1/2" x 5" long. There is a large plug on one side of the box.

3) Pull the cable with plug out of the relay and you will see six socket connectors in the plug. Set up an ohmmeter that can read very low values. It must be able to read 10 ohms full scale. Connect one end to one of the socket connectors in the plug and connect the other end to a good ground. The reading must be at 1 ohm or below. Repeat this test for all six glow plugs.

You will probably find one or two that read infinity. The plug is open and provides no glow.

Replace any plugs that are above 1 ohm.

Report back on your progress.

Good luck.