I just replaced my positive and they offered like 3 sizes for my year mustang - I got the 20" and it's a little long. 16-18 (18 if you wanna be safe) would probably be sufficient, but it's notl ike 20" isn't good, just a little longer than necessary.
I'm not sure about the negative cable. The auto parts store should be able to look these up for you on their computer, though. BTW, get 4ga cable, not 6.
I'm not sure about the negative cable. The auto parts store should be able to look these up for you on their computer, though. BTW, get 4ga cable, not 6.