Finally, I can contribute and stop feeling like such a tool. I just did this addition a week ago, so I'll gladly answer your question as best as possible.
I purchased a block heater for my 1991 M925A2 (Cummins 6CTA8.3 engine). The heater type I acquired is a freeze plug type, 1000W, 120V (will pull 8.3 amps). It fits in the 2-1/4" freeze plug located on the passenger side rear of the engine block. Available through any NAPA, Kat's model 30108. Comes with the cord, about $65 if you can get a NAPA discount. NAPA can also reference this part number, or here is the NAPA link:
http://www.napaonline.com/Catalog/C...ug-Type-Block-Heater-/_/R-KAT30108_0282404207
The heater is also listed in the Kat's catalog as specified for the Cummins 6CTA8.3 series engines on page 32 of the Kat's catalog (or page 36 of the actual PDF viewer):
http://www.fivestarmanufacturing.com/getdoc/2c8a6911-c214-4190-9059-d6aaa10fe527/DC12.aspx
Installation is as follows, and less than 10 minutes after you have the coolant drained:
1. Drain radiator coolant via petcock at lower right of radiator. Expect about 6 gallons to come out.
2. Remove right wheel well splash shield.
3. Start engine briefly to turn wheels full left, then engine off.
4. Using a long pry bar, place the tip inside the cup portion of the freeze plug at the outer edge. Hit the pry par with a hammer 1-2 times firmly, and the freeze plug will flip 90 degrees in the block hole. Reach in, and pull the plug out with pliers. This plug is steel, and will likely not be damaged at all during removal, and hence can be reused if need be. Mine came out unscathed.
5. Using a drill with a small wire wheel attachment, run the drill at high speed around the inside diameter of the now open block freeze plug hole. This will remove all of the sealant which is used from the factory to seal the freeze plug in. Be picky here, and make sure you run the wire wheel until this bore ID feels as smooth as a mirror. Any edges of sealant left may cause the heater seal to leak. This hole on my block had enough hardened sealant in it to prevent the heater from even seating fully in the block.
6. Remove "O" ring seal from new heater.
7. Test fit heater element into open block freeze plug hole. The element will only fit 2 ways, with the cord connector rear or front most toward the engine, due to the protrusion of the actual heating element making contact inside the block. Take not of this for actual assembly, and remove heater.
8. Apply grease, silicone, or assembly lube to the heater "O" ring, and reinstall it to the heater.
9. Insert the heater into the block hole. when you push firmly and squarely, it should seat with a firm action. Snug tight the hex head bolt, you don't need to kill it, as it is simply keeping the heater from blowing out when the cooling system is under pressure.
10. Connect the cord, and zip tie it out of the way of the exhaust manifold, route the cord end to a place you desire to store/connect it when needed.
11. Refill with coolant. You may need to go for a short drive and refill again as it burps out air. My heater blew cold yet engine temp was 160F at first, indicating air. A second refill of coolant brought everything back to normal.
Attached is a picture of this unit, as installed, to help display the location and orientation. It is just above the starter solenoid in this photo:
It took longer to write this than it did to actually install this heater.
