I have a 2005 and yes i'm sure it's a 3157 bulb.
The 921 is similar to a 194 which are the license plate bulbs. So, that can't be right.
The backup lights are 921, on gen II at least.
It seems very unlikely that anything would use a 3157 as a backup light. They're generally used for brake lights and turn signals, since they have two filaments.
Oh yea? Well I appreciate and i apologize for knocking you man