For a 2008 it should be under the bed.
Late 2008 we started a running redesign, so if you have the heated enclosed tanks, it will be located in the compartment 2nd from the rear on the passenger side. Open that compartment and there is another little black door, it is behind there
PLEASE do not send questions via Private Message, my inbox fills up too quickly. Post questions in the forum as a new thread so that other users can benefit from the answers. Use Private Messages for sensitive information only (VIN, Contact info or things you do not want the public to see).