There are two general types of car seat warmer models: in-seat models and external models. The external warmers sit on top of the seat and can be removed at any time, while in-seat warmers are built into the seat itself and can be turned on and off but cannot be removed easily. An external car seat warmer is essentially a seat cover with heating elements inside, often controlled by a hand controller and powered by a connection to a 12 volt outlet in the car. In-seat models are powered by the car's battery in many cases, and the controls for the heat are built into the dash or center console.
Choosing the best car seat warmer for you is a matter of deciding how much you want to invest in the heater, how often you will be using it, and how comfortable you are with your current seat configuration. This last consideration is important because an external seat warmer in the form of a seat cover will affect the cushioning and comfort of the seat, since you will be adding a layer of material and cushion between you and the seat itself. Some people prefer to avoid such models because of this fact, while others are comfortable with extra padding on the seat.
If you choose a car seat warmer that is an external unit, be sure to choose one that secures to the seat tightly enough that it will not shift too much during use, causing potential discomfort. It should feature a hand control that allows you to adjust the level of warmth and to turn the unit on or off quickly and easily as well. More importantly, the unit should have an automatic shut-off feature that prevents it from accidentally overheating, which can be a fire hazard.
In-seat car seat warmer models can be installed relatively easily if you are skilled with basic tools and are familiar with electrical work in a car. The battery should always be disconnected during installation. Look for kits that install easily, and ones that are within your budget. All necessary wiring should be included with the kit, and any means for fastening the heating pads to the seat should be included as well. Look for pads that are wide or tall enough to cover a large swath of the seat to ensure even and sufficient heating of the seat when in use.