01 Expedition Premium radio sub addition

I'm new to this forum and have been searching for an answer to my question, but cannot find what I need.  I sure wish this sort of resource was available when I was installing and competing back in the early '90's! What a wealth of information!

OK, so much for the brown-nosing ;) ...I want to hookup an amp and sub to the factory Premium (not MACH) sound system (there is not a factory sub installed, and it is not wired for one either) in my 2001 Ford Expedition. Where is the best place to get the signal to send to the sub? Non-fading would be nice, but I can't see how that would be possible. I have the power wiring issue figured out - just need to know the best place to tap into for the signal and not lose any of my door speakers.

First I would verify if you have an amplifier in the vehicle.  If you do not the I would go behind the radio and just tap into the speaker wire outputs of the stock radio.

If you do have an amp then I would go in the kick panels and catch the front door wires.