Probably only a mystery to me because of my inexperience buuut....

1) Car was taking a lot of cranking to start, was getting fuel ok, so I checked plugs, only one was functioning.

2) Replaced all four glow plugs with cheap Autolites, there was so much carbon buildup I could barely get the plugs seated to thread them in.

3) Honeycomb was taking a very long time to glow orange still, and even then was not getting nearly as bright as it should. Car was still taking a ton of cranking to get started.

4) I bought a reamer, reamed the glow holes, reinstalled the Autolites. Long glow time/half power persisted, until one time when I was trying to start the car I heard two quiet 'ticks' from the engine, honeycomb flared extremely bright (I instantly reflexively let go of the crank pull), and now the honeycomb won't light at all.

I'm at the end of my understanding of the glow system. All my fuses in the fuse box look good (I don't even have one labeled 'Glow Plugs' anyway, I assume if there is a fuse it is elsewhere). Could it be that the cheapo Autolite plugs were shorted out or otherwise damaged when I tried to use them in the heavily-sooted pre chambers? At this point, I don't know what else to do except buy better quality glow-plugs, install them, hop on one foot in a southward facing circle 720 degrees chanting...).

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1969 220D - 4spd std
1973 220D - 4spd std

has the car been fitted with the newer style pencil type glows or is it still the original equipment style with the wire loop? Are the glows connected with open rigid wiggly wire or with insulated wire that bends?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Old style wire loop plugs, rigid wiggly wires.

Tried to upload a photo, but no dice on mobile. Will when I get home in the morning. The wiggly wires are as you say, uninsulated with ceramic insulater spacers. It's all stock 69' 220D.

You won't like me saying this but honestly loop plugs are not worth the hassle, just find a set of the pencil adaptor plugs and be done with them, they should've been phased out 30 years ago...

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

NZScott. I might do that down the road, but right now I just need to get the old girl rolling. The laundromat is too far to walk and I can't just keep buying underwear on Amazon forever.

I think you have solved your problem. Buy decent plugs. Ream the holes. Meticulously clean all the electrical contacts during installation with fine sandpaper or steel wool. I use electrical grease on the contacts to ensure a good connection over a long time. Don't over torque the new plugs or you will break them. As an aside, there isn't a fuse for the GP system.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Damn. I have reamed the pre chambers, cleaned all connections, and installed brand new plugs again and nothing. The salt shaker takes 30 seconds to even look dark orange (whereas it used to be brilliant glowing orange in 15 seconds), and the car won;t start no matter how long I glow. I don't know why else to check.

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1969 220D - 4spd std
1973 220D - 4spd std

Have you chcked for voltage with a milti meter at each plug?

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

So the fact that the salt shaker eventually glows is an indicator that your entire circuit is, in fact, working. I had similar symptoms after changing my plugs for the first time because of excessive resistance at each connection. But it sounds like you have cleaned everything now.

Have you checked the ground strap between the first plug and the engine block? Maybe it has a poor connection which is adding resistance to your circuit.

Using a voltmeter as mentioned previously is another great next step. Have someone hold the gorilla knob while you measure voltages throughout relative to the engine block. That will give you a hint where the excessive resistance must be. I think you are expecting about a 1 V drop across each GP and about 2.5 volt drop across each wiggly thick connector.

I personally love having the old school GPs, but if I was in your situation, I might get rid of the existing system and start over with something new.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Not trying to be smart, but if you ordered those modern plug conversions back then you'd be happily driving down the road now without a care in the world. Remember you might fix the system now, but it is a ticking time bomb before a plug fails and then you're back to square one...

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD