amp
same here. i have an amp under the backseat along with two subs and i also have an amp under the passenger seat.
on the 08's ive found it easier in the long run (shorter cables, too) to run the wires through the passenger side of the fire wall, right behind the battery. Its a tight spot to get your hand into, but doable... trust me I just did it this week 3 times lol (an 04 FX4, 08 XLT and 05 XLT) had great luck. run the wires under the fuse panel and down the pass trim molding, cant go wrong!
on the 08's ive found it easier in the long run (shorter cables, too) to run the wires through the passenger side of the fire wall, right behind the battery. Its a tight spot to get your hand into, but doable... trust me I just did it this week 3 times lol (an 04 FX4, 08 XLT and 05 XLT) had great luck. run the wires under the fuse panel and down the pass trim molding, cant go wrong!

