hi ,i have a 2001 ford ranger 4x4 edge ,nice truck, i installed 2 12' subs,a kenwood 600 w amp and a capaciter to keep enough power stored to run amp ,i kept factory system in the truck and wired the amp from rear speakers heres the problem when i get in the truck and start it i dont get any sound unless i open a back door or passenger door,once it is on im good until i shut the truck off again any suggestions thanks