A better service to consider that is less-competitive and easier and faster to perform would be carpet recoloring and dyeing with a system that has the dyes color matched to the 16 most common automobile carpet colors.
There are great systems out there that have you in the business immediately with no hands on
training necessary.
HEADLIGHT RESTORATION
Another would be headlight restoration. Sanding and polishing oxidized and faded polycarbonate headlight lens. Systems as low as a couple of hundred dollars. You can charge as much as $50 per headlight.
These are far better alternatives than competitive and complicated window tinting.
Bud Abraham