Baby seat/deat belts

I really need some help figuring out a safe and legal way to be able to bring our two month old daughter Alanna to cruise nights in the Demon, I'm a little skeptical about the seat belts in the back seat . did anybody else ever run into this problem or does anyone know a good place to purchase after market rear seatbelt so i can strap her car seat in without being worried that the belts will fail?
Help this fantastic chick see some Detroit muscle!