Since neitherj will recast thwe remaining balance (they will both expect the same full payments until the loan balance it paid in full), I'd give the money to the auto loan.
Personally, I'd throw the money at either my savings account or my credit card(s).
I don't see much advantage in burying money in the equity in my house because if I want the equity out of it for any reason, I have to qualify for a new loan. There is nothing "wrong" with having money buried in your house, I see no point in stashing cash there. Stash your cash in a more liquid location for access if something arises that is not foreseen at this time.
If you are not planning to keep the car until it completely falls apart -- most people keep a car something like 5 years on average, and lots of people keep a car less than that -- I see no point in throwing extra cash in that direction either. Driving a car that is paid for is always a good feeling, but if you are the sort that trades a car as soon as the last payment is made, then throwing your extra cash at a car loan might not be the best thing.
For these two reasons, I'd stash the extra cash in my savings account, or in my credit card balance. Having said that, my credit card balance is zero so perhaps my outlook is a bit skewwed.