2003 Expedition, Premium Sound, Amp + Sub

Im going to be installing an amp and a sub in a 2003 ford expedition soon with the the premium sound system ( mach system ). Whats the easiest way to install this? I told my buddy he needed a line output converter, i remember i put an amp and sub in a ford mustang once with the mach system. I installed the line out converter behind the radio at the speaker wires but found out that it didnt work and ended up installing it at the rear speakers. Is the expedtion the same way or is there an easier way? Also is the factory amp under the rear seats or on the passenger side rear quarter panel?

thanks for your help

thanks but i got a few more questions. He says that he has installed aftermarket speakers in the doors and in the rear doors. He was saying that tweeter installed them. I asked if they bypassed his amp or what not. He says no. shouldnt they have bypassed his amp? Whenever i put in speakers in premium/ mach system i would always tell my customers if you decide to upgrade your speakers get a 4 channel amp to power them because i have heard stories about the ohm load of the speakers not being compatible with the factory amp. Just to be safe get a 4 channel. anyway, he says he upgraded all his speakers to some sony speakers.

i sat in his truck the other day and i didnt hear his factory sub. maybe its blown or disconnected im not sure. Is there a different way to add a sub instead of using that soundgate part? I want to give him a couple of options in case he doesnt want to buy a $92 part.

You can use the cheaper LOC from PAC # Trunk-LOC or SoundGate's LOCA

As far as the installation from Tweeter's go, make sure you get the full details from the installer, not the customer because most of the time the customer doesn't always know the full story or the installer doesn't tell them the full story about the installation. Chances are the installer did bypass the system and removed the OEM subwoofer from the system wiring.

also wouldnt a line output converter from best buy also work in place of soundgates LOCA? i know they have them for like $20. i hate doing side jobs for people when they are your neighbor. i got to take another look at this guys truck to see if his amp has been bypassed. i dont want a car with boobie traps and such.

thanx for all your replies though