If you have a lot of money to play with...you MAY be able to increase the DOWN PAYMENT to get the interest rate you want...
Not sure with regards to your ??? But, my dad used to work on the final line at GM and he said they used to come down all the time and tell them to put on new stickers (with a higher price) "We got a promo..let's change it up boys" So, just my opinion, shop around at various GM dealers for a price you want and some negotiable terms before you sign and make sure you read everything when you get back there with that slick a$$ finance guy (who has probably changed all of your terms). He gets paid to sit there. Make him wait until you are finished reading. And I will say it again...Don't let them charge you the DOC fee which is usually about $200-$300. This is a BS fee and law says that they can no longer charge the customer for what I call their petty cash fund.
Oh yeah, and don't let them bounce you around from financier to finacier....The inquiries suck. And no they cannot do it through Nuvell (aka GMAC) - too high interest anyway. Tell them if they can't do it GMAC don't run you again with anyone else.