Unless a manufacturer is having a special, like many of the auto companies where they offer below market rates or even zero percent, you are usually better off to arrange your own financing versus using a dealer. The dealers don't arrange loans for free, they add on to the true rate they get from the lending source and pocket the difference. One of the best sources of lower interest loans is as Haird mentioned, local credit unions. Other options like a secured loan can result in below market rates as well. By a secured loan I mean a situation where you have an account with an institution with $XX in it. You arrange to take a loan and use that account as collateral, rather than the vehicle. This used to be a fairly widely available option, I have no idea if banks and credit unions still offer it these days.
As for using home equity loans or second/third mortgages, many people have done that over the years but I have a philosophical problem with that approach. I don't believe in risking your home over something like a car, or even worse a toy like a motorcycle or boat. As millions of people learned in 2008, stuff happens that is beyond your control. It's much better to have a toy repossessed than it is to have a home foreclosed.
JMHO - Jerry