I run a timer also. Mine kicks in 3 hours before I leave. Earlier if it's really cold. It costs me roughly $0.08/hour to run my double heater set up. I bet I save at least $1 on fuel every day by starting with a warmer engine that doesn't have as much unburnt fuel going out the exhaust when first started. It also takes much less time to idle down. I always let it idle till it idles down before I drive it. (4 minutes in summer after sitting overnight, about the same if block heaters plugged in during the winter at -25*, 11 minutes or more on a cold start at -25*. if it's +25* and both heaters are on, it doesn't high idle. No glow plugs wait light either.)
Recovry: "Front freeze plug on driver's side" I was told rearmost driver's side or middle on driver's side. The middle on the driver's side would be easiest to get at. If I were to do it again - I would put one in the middle on the driver's side. Mine is in the rearmost driver's side. Works fine.
On M1009's (and I expect M1008's, etc.) the front freeze plug hole is covered by part of the engine mount if I remember correctly.
Warwag: I agree, the wrong magnetic heater can overheat/break down the oil.
Also, for those considering oil pan heaters - they do not heat the engine block, just the oil. Not very much heat gets transferred through the oil pan. If it's cold - heating the engine block (freeze plug heater, in-line heater, etc) is what you need. Yes, warm oil is a good thing. Yes, it makes everything turn easier. No, it does not help the compression temperature like heating the block and cylinder heads, which is what a diesel needs to start.
Circulation heaters that heat and pump coolant around through the heater hoses work great where it does not get super cold. They do not work well where it gets really cold. This is because the coolant passes through the heater hoses, heater core and/or radiator before getting to the engine block. The hoses lose huge amounts of heat when the surrounding air is 40 below. They waste alot of energy, not much of the heat they make actually gets to the block when it's super cold outside.
Freeze plug heaters put the heating element INSIDE the block. That is why they are the most efficient. The coolant gets hot, moves over an inch, and warms the block without any heat losses in hoses. It then rises with convection and heats the heads also. The thermostat, being closed, prevents the hot water from flowing into the rad (where it would just waste heat into the air). My coolant crossover tube gets warm to the touch. My rad hoses stay cold.
Lower rad hose heaters work in a similar way to freeze plug heaters, but they do lose heat in the few inches of rad hose before the warm coolant gets to the engine. They also lose heat themselves, as they are little doughnut shaped metal things out in the cold air, compared to the freeze plug heaters which are completely surrounded by coolant and the engine block. Also, convection does not move water super fast. So even tho the hose between the engine and heater might only be a foot long, the coolant might spend 30 seconds moving through that hose. Coolant spending 30 seconds in an uninsulated rubber hose surrounded by -40* air loses alot of heat.
Personally, I would choose a Block heater for any vehicle in this order:
1. Freeze plug heater.
PROS: Most efficient. Puts heat right where it's needed. Can put in multiple heaters if it's really cold. Usually quite cheap. ($30 each for the two 600 watters in my 6.2) Not hard to put in, needing only a metal rod, a hammer, pliers/vice-grips and a screwdriver. Not likely a fire hazard.*
CONS: Harder to put in than other options. Need one that is specifically made for your engine.
2. Lower rad hose heater.
PROS: Super easy to install. More efficient than circulating heaters. Don't need a vehicle specific model, so long as you get one the right diameter for your rad hose, it will fit. Will work good so long as it's not super cold. Not likely a fire hazard.*
CONS: Less efficient than freeze plug heaters. Can't really install more than one (on most vehicles). Two more rad hose connections that could blow off on the road if not kept in check.
3. Coolant Circulating Heater.
PROS: Even easier to put in. Will work if temps do not get very cold.
CONS: Very inefficient. In my experience, you will need 1500 watts or more to heat engine block anywhere near as well as a 600 watt freeze plug heater. Can restrict flow to heater core (less heat in the cab while driving). Expensive. Must mount metal tank somewhere under the hood, where it doesn't melt anything. In my experience, they don't last very long. Small water Pump = more parts to break. The pumps are tiny and can be clogged by little bits of rust and scale floating around in your coolant. Depending on the vehicle, the thermostat might need to be open (engine hot) when you turn on the block heater, in order for it to work. I seriously doubt a 1500 watt heater has enough power to heat even a small 6 cylinder engine to a hot enough temperature to keep the thermostat open (~170* depending on vehicle. "cold weather" t-stat on my M1009 opens at 195*).
I suspect that these style heaters were originally intended for tiny 4-cylinder engines, not our Dodge 318's or 800+ pound GM 6.2's. A friend of mine has one on a VW 4 cylinder diesel and says that it works ok. It sucks 1500 watts. And works as well as the 600 watt heater in my old man's minivan that has twice as big an engine.
*Freeze plug and Lower rad hose heaters are completely surrounded by coolant. This makes them rather safe, because if something goes wrong inside the heater, it'll just burn itself out. (All block heater power cords are potential fire hazards and should probably be checked often to make sure insulation hasn't been rubbed off, potentially causing a fire) Separate-tank style heater-hose circulating heaters pose more of a fire hazard, as they have motors and electrical that is not surrounded by water. Also, if there is a leak/the pump breaks, they can run dry and catch fire.