[bluGill]

Also assembly lines can achieve the scale factors needed to pay for all the jigs needed. A dealer doesn't sell enough cars to afford those expensive jigs. Making your cars in the factory is nearly always going to be cheaper the dealer customization. The dealer can do minor things that are quick and easy, but putting a different back on the bed on is something the factory should be able to do better and cheaper.

[panick21_]

Ideally you would design it perfectly self centering, fool prove install and so on. So that the cost at the dealer would be small and quality high. But that's easier said then done, specially if you move deeper into the available features.