I love this game so much, been playing it for a long time now. If you activate the map extension, you can press "M" for your map... Upholstery store is at the top left marked with a shopping cart.... It's a very risky and long drive there, so be very carful if you do decide to go there.... Keep up the videos dude! You doing great!
Only way I know to change the interior colors is to go to the junk yard and look for panels in your color. They can be removed from the junk cars there I have not ventured to the upholstery shop yet