2001 ford explorer w/mach line level?

I did search before posting and couldn't find much regarding this. I attempted to hook my Fosgate 1500w amp and two Fosgate 10" subs to my factory H/U using a line level converter. However, it appears that the H/U has a built in crossover somewhere in the range of 150hz. My question is, where would be the best place to tap into the factory system? I want to keep the stocker for now (and intend on replacing it with a navi-unit when money permits me to buy a GOOD one). If I tap into the factory sub's output, what is the cutoff on that? I want to be able to adjust my frequency manually, so is there anyplace on the system that is providing a full range signal??? Thanks so much!!!!

One other thing I have in mind. Since I won't need to use the factory sub, can someone tell me if the head unit sends a low frequency to the amp or does the amp have the crossover built in? If the head unit DOES send a full range signal, how would I tap into that? I am presuming it is a 2V-5V signal much like what would output on a standard RCA connection? Or is it something different?

You can try at the back of the deck, but the signal might not be enough depending on the LOC that your using. What's the input sensitivity of the LOC that your using?Jeff

Velocity Custom Home Theater

Mobile Audio/Video Specialist

Morden, Manitoba CANADA