Aftermarket Stereo Problems (amp/sub)

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PLEASE HELP!!
I have a 2001 mustang conv. GT with the mach 460 sound system. I have hooked up a sub and amp to the factory head unit. I spliced the line level converter into the speaker wires between the rear mid range speakers and the factory amps to get the signal to my amp. My problem is that I am getting a signal to my amp but it is so weak that the sub is barely driving. I've tried many things to fix this and have had no luck. I used the blue/white stripe wire on the head unit for the remote switch for the amp that all works fine. My last attempt is going to be to try and grab the signal further up the line before it runs through the factory amp, but I have no idea what color wires are for the rear speakers. Could someone please shed some light on the situation for me? Thanks