On my 91 2.5 602 engine i have had exactly that problem a couple of times.No glow light until a bit after the engine starts and then the light comes on for 30 sec or so,and both times it has been one bad glow plug.I was able to determine which one by testing with The DMM,your test numbers in post 5 look normal,try that test again.If they still all seem normal one sure test is to remove them and apply 12 v with jumper cables to see which ones dont get as hot as the others but thats alot of work just to find one bad one.If you have access to an ammeter that can read 30 or so amps you can test the current draw to each plug one at a time by connecting 12V + to each socket in the plug that plugs into the relay one or more will probably not draw much current those will be the bad ones. They can be replaced without removing the manifold if you have a 10mm universal 1/4" drive and a 12mm deep 1/4 dr and a 1/4 u-joint and or wobble extension and some patience.
the nuts that hold the wire to the plug are captive in the wire end connector so they dont fall and get lost.Some anti-seize on the threads of the new plugs is a good idea too.
Hope this helps Don

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