It has a resistance of 12ohms, while the others are at 1.0 to 1.7ohms. I am going to change the one at 12. It was terminal #2. Any idea what glow plug this is? #2 would be too easy.

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95 E300 Diesel 200K

It should be #2. You can easily verify that with a meter. I would probably attempt to change them all, since the intake must come off. Just be sure to not put too much torque on them. If another one doesn't want to move, you might want to leave it alone. Hopefully #2 won't be the one that won't come out.

Good luck. These get real stuck in there. If you;re measuring at the junction, it might be a bad glow plug wire , not the plug, so check the plug.

If any of the Glow Plugs are removed you might consider reaming/cleaning out the Glow Plug hole as Carbon build up is one of the reasons that they get stuck in the head and applying a Never-Seeze type compound to the threads when you install the new one/s.
I would also read some threads on the removal of the Glow Plugs as the longer ones from your vehicle often get seized in the threaded area, from Carbon buildup around the tip or a combination of both resulting in it/them breaking off in the Cylinder Head.

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84 300D, 82 Volvo 244Gl Diesel

What tool do you use to ream the glow plug hole?

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95 E300 Diesel 200K

There is an actual reaming tool available.......but I have just used a drill bit on all my past and present diesels. You spin the drill bit with your fingers to loosen and draw up the carbon stuck to the wall of the plug hole/shank opening.

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K