Your question was about needing a cosigner and you get advice on if you should buy used or not.
My answer, yes, more than likely if you needed one the first time you applied. The reasons the finance company wants a cosigner can be, you don't have established credit yet or maybe you've defaulted on a loan in the past or filed bankruptcy.
If you can finance something for 0%, "it's always a good thing to use someone else's money". My wife was in banking for years and that has always been her motto. Just make your payments on time and a little more than the minimum so you pay it off on time and do not get hit with finance charges.