I ordered one installed from the factory and it was on my invoice but I forgot all about it. Where is it located on a stock JK?

 

Look for the wire coiled up in the area of the battery or fuse box.

 

This is a whole different type of heater - this uses the vehicle's own fuel supply to het the engine coolant, much like a "pro-heat" on a commercial diesel truck. This offers faster heating and you don't need to plug it in - but you also pay a premium for it.

Most North American passenger vehicles that require block heaters run a 120 volt block heater, and maybe a battery blanket as well. If you're going to spend your winters in a cold environment, it's definitely worth installing a block heater.