The kit part came with the floors as plain plastic and some nicely molded rubber mats to affix to the floor with some double-sided tape. I wanted to carpet the floorboards so it looked closer to the real thing.

This image was found from GBFans.com’s Ecto-1 restoration gallery.
I initially got the idea from a mod set available from Mike Lane Mods. He has a front carpet set that looks great, but for someone in the US, it’s a bit much with the shipping. So, I did my own.
In looking, I found a lot of examples of people using a flocking method of making carpet for models. The scale of the flock material will determine the shag of the carpet in the model. Given how large this 1/8 scale model is, I decided to go the super easy route. I got some thin craft felt at Michael’s and used double-sided tape to apply it. I recommend making a pattern of the shapes by using plain paper, and trimming it with a blade. Then you can apply the templates to the felt.

Make sure to cut out the places where parts will affix using the rubber mats as a guide. There are the slots where the seat will screw in, and where the accelerator pedal and high-beam button attach.
One think I went back and changed was that I added material to cover the plastic still visible behind the seats. This is visible when the seats are tilted forward.
I like the mats that came with the model so I’m still using them, but over the carpet like they would be in the real car. This is also a more easily weathered material than the felt. I do think the black felt needs some weathering though. I don’t intend to have a showroom clean car.