"In ecommerce, the shopping cart is the boss ...."
If you mean by the sale is ultimate boss, I agree. but as you mention, the business is a team of various parts and pieces. So, in reality, I don't the boss isn't the shopping cart unless you have a monopoly on the market and don't care what reviews say.
"Dealers think leads are the shopping cart."
Dealers shouldn't think that leads are the shopping cart, unless you are considering these leads as an item in the cart.
If your cart doesn't have an item, there is a problem with abandoned items or with the boss.
So, I see the cart as the finance office which I should really say is a payment gateway. But why split hairs, right!
Basically, you will need to review all your teams to find out where in the checkout process something broke.
Your likely candiates are:
the boss - you got to get leads
the cart - you got to get the leads to commit
the payment gateway - getting the customer to actually sign up for 84 months of financing.
Each part plays an integral part on the team!