I know I'm going something really dumb, but I just don't see it. Does there seem to be anything wrong with this little bit of code?
$sql = "INSERT INTO cars (users_user_id, car_make,
car_model, car_year,
car_engine, car_description, cars_cmdos, cars_fmods)
VALUES ('$user','$make','$model','$year','$engine','$description','$cmods','$fmods')";