The 6.2 forums at
http://www.dieselplace.com/ have a good write up in there some where.
The way the original system works, it's a timer relay with an adjustable rheostat that senses engine temperature and adjusts glow time based on that. Colder, longer glow. Two problems, the self lmiting glows heat up a little slower and so need more on time, and the relays fail a lot, particularly the round first design. On mine, the relay always clicked, but glows did not always seem to work. I wanted to upgrade the glows anyway, and the non-swelling ones needed more glow time, so off I went.
You will need a Ferd starter solenoid, about $16. A momentary switch (push button or toggle, you choice), $5-15. Some 12-16 guage wire. Some battery cable (more on that in a minute). And most importantly a set of self-limiting glow plugs, mine are AC 60G's, about $80 delivered from that big auction site.
First, replace the glows. Clean, use some PB blaster, and hopefully they'll come out. They're metric, by the way, and a3/8" deep well works.
Now figure out where to mount the solenoid. On my 87, I used the rear leeft intake manifold studs, tossing the EFE/EGR solenoids and brackets on the way by (since I replaced the intake with a non-EGR one). I had to file one of the holes in the mounting base slightly to get it over the studs. Just remeber the solenoid base plate MUST have a good ground!
Now run a wire from the solenoid terminal to the switch, and a hot wire (fused of course) to the switch. In my case, I used a hot in run connection in the fuse block, mounted the switch in one of the holes on the bottom of the dash where it's easy to reach but not real obvious. I ran the wire to the solenoid out thru an existing grommet next to the steering column.
Many people remove the original relay. I chose to leave mine in and jumper around it, using it as a terminal block. So from the hot terminal on the old solenoid, run a feed to the HOT side of the new solenoid, then run the new solenoid's out to the glow plug side of the original. So now the solenoids are in parallel. I used some 4 guage lawn mower battery cable (a little overkill but these things draw a LOT of current, and I had it laying around...), with 4 crimp-on ends that fit the two solenoids. When you route these be sure they don't rub on anything (700R4 TV linkage and air cleaner were my issues).
When I turn the key on the original solenoid clicks, I reach down and hit the momentary switch. After some period of time the original clicks off, I hold the switch for another 10 seconds, release it and crank. The beast always fires up. IF the soenoid doesn't click I just count, 25 secs below freezing, 15 seconds for summer days or fully warm engine, and the rest im between. By holding the switch while the relay is on I don't have to worry about whethere it is functioning, I'm just using it as a crude timer. Sometimes it's doesen't click in and then I count my own time. The nice thing is the after start glow function still sems to work.
Hope that helps.