There are a few different types you can by. One mounts in line with the lower radiator hose and needs to be spliced in. Another type is one that has a holding tank and splices into your heater hose. There are also magnetic mount block heaters. And lastly there are heaters that install in place of a freeze plug.
I chose to install the in line lower radiator hose heater. This will be the second one I have bought. The last one was on my old Caravan. Worked flawlessly and warmed the motor perfectly in harsh cold weather. I have plugged it in overnight many a nights without a problem. I like to instal mine on a rising section on the engine block side above the lowest spot of the radiator hose, not at dead bottom.
This tends to create a very slow circulation of the engine coolant. Because as the coolant in the rising part of the hose warms and rises up, it is forced to go up and heat the engine block. As it does, the cooler coolant from the bottom of the hose and the radiator flow towards and up through the heater block.
Here is the one I chose to buy, I got it of eBay at this link for $19.99 plus 4.50 shipping out of Syracuse, Ny.: