I bought a 2016 Fit LX a few weeks ago. Leather was a must have for me, but I didn't like all the extra junk that I was required to purchase to get leather from the factory (and about a $4,000 price increase), so I decided to get Katzkin leather after purchasing, and I decided to get the Katzkin DegreeZ heated and cooled seat for my drivers seat (https://katzkin.com/degreez). The leather cost $1,000 (installed), and I'm very pleased with it. The heated and cooled drivers seat cost an extra $700, and I'm not very happy with it becuase the main reason I wanted it was for cooling (not heating), and the cooling is very weak. The heating works well, but I'm almost never going to use it. Also, Katzkin sells the heated seat without cooling for a much lower price (but the cooled option includes heat and is expensive). Also, its worth mentioning that most dealers will sell you the Katzkin product with the new car so that you can finance it (the dealer just takes your car to the same installer that you can go to on your own). I went straight to the leather installer because it saved me a few hundred dollars of dealer markup and I didn't care about financing the leather with the car.

Here are some pictures of the new interior: https://goo.gl/photos/zGtbD2Khk2E3gbUq7

 

I've owned more than one. -with more than one vehicle.
The one i have now is a Homedics, that was designed to be used with a 12 volt, for in the car, or with a plug-in for home/office use, and it actually has not only heater elements but also a "massage" option. I have used it for both.

The massage option, I think makes it a little thicker than just your normal car seat pad. So I'd say an inch to two inches at the thickest.

I have owned and seen ones designed to ONLY heat, and they are much thinner.

PS.
I'm NOT against people doing the project. Like I said, I think heated seats are nice. But there is a less DIY, utilitarian option, for those that don't want to get into wiring and upholstery adaptation.
And the end result....is pretty good IMO.

 

In the past I have simply trimmed a piece of quilted aluminum insulation (it has air pockets) and sat on it! After a minute my body heat bounced back...w/o altering the car. Something like looking into a mirror to tell how handsome/pretty you are.

 

Where do you buy quilted aluminum insulation? How thick is it? Home Depot, fabric store?

Interesting idea, and sounds similar to insulated boot insoles, which are aluminum covered.