My husband is looking to purchase a 2005 GMC extended cab sierra. We have a 18 month old rf, 5 year old in harnessed booster, and a baby on the way. I currently have the following car seats: Britax Marathon (in main car for 18 month old), Costco Scenera, Graco Naultis, Britax Frontier (in main car for 5yr old) and the baby will be in a Chicco keyfit. Does anyone know if any of the car seats will fit? Or if I will need to purchase different ones?