The "dormant commerce clause" power is actually the last bit: B/C the power to regulate interstate commerce is given to Congress, it is not within the domain of the states to interfere with (i.e., by regulating) interstate commerce.

The dormant commerce clause power doesn't prevent states from having incompatible vehicle compliance measures...the commerce clause does b/c Congress has passed legislation on point. States are allowed to have incompatible vehicle standards (Cali and NY/NJ can set their own emissions and fuel efficiency standards) b/c the legislation allows them to seek waiver. If the dormant commerce clause applied, they wouldn't be able to do this b/c there wouldn't be any waivers available b/c Congress was silent on the issue.

IAALBYANPM.