Let's see if I understand it...
On the other hand, as you pointed out, Aunt Tillie can give you money if she chooses to grant you an unrestricted loan. In that case, the money IS fungible. You can invest in whatever you want with that money.
Bottom line, the only thing you can "invest" in with a car loan is a car. That's why it's called a "car" loan.
Let's see if I understand it...I traded in a lemon and used it as a down payment. I am paying 3.9% interest on the roughly $20K balance. If I had put up the $20K I wouldn't have it liquid. If I have it liquid I can invest it. No one (including me) knows whether the $20K I invest "comes" from the loan or the myriad of other that pour freshets, streams, and mighty rivers of $$$ into Wattempire.
Ergo: I can invest it.
Correct? (You seem to know much more than I about the topic.)