adding another button fopower door locks

i have an 03 yukon all factory with keyless entry what i am trying to do is install a button at the rear door (lift gate) to lock the doors how can i do this ? the only thing i have figured out so far is the is a lock relay in the fuse box at the drivers door when i tried to use this relay to lock the doors it only locks the rear 3 doors, does not lock the front 2 doors i was also recommended to buy this http://www.sonicelectronix.com/item_28967_XPressKit-GMDLBP.html?SearchClickout[query_id]=46729808&SearchClickout[Ignore]=1 the company that recommended this could not guarantee it will work for my application and wasnt sure for sure how i should wire it up i called the selling company and they offered no tech support thanks for any info

OOPS topic should be adding another button for power door locks