Reading the mod page carefully answers the question about the roof lights:

Unfortunately, not everything can be done with your own hands, so wandering around the stores will help you find the following improvements:
roof lights to mount on the roof in a circle,
chains on the wheels,
snorkel.

In other words: you'll have to find the roof light item.

As for the 6 seats: The car itself only has 4 seats. if you attach athe tent rack, you gain two more "seats". But 1) you can only access those seats if the tent is folded up and 2) you can only access those seats from ouside the car.