My Mustang has the optional Block Heater. It's a Massachusetts car.
Plug is located int he LX grill off to the side. No real special holster or anything, it's just hanging out there.
The wire runs down the passenger side fender and under the passenger header
One of the freeze plugs in the block is removed and swapped with the block heater plug. It's just a thicker freeze plug with the attachment to plug the wire for the heater in. WHen you plug it in, it gets pretty warm and does a good job of keeping the coolant warm at night.
I've used my block heater quite a few times this past winter when we had 2 weeks straight of dipping down near -20 at night and highs of 3-5 degrees.