2004 Dodge Ram amp addon?

I have a 2004 Dodge Ram with the 7 speaker Infinity sytem. I know that it uses an external amp behind the glove box for the speakers.

I'm going to add a sub and amp to the stock system. Should I tap in after the stock amp for high level inputs, or would it sound better if  I wired in rca's to the low level wires before they get to the factory amp? 

This depends on the type of interface that you will use. Most LOC type devices are only designd to work with a post amp signal. Some, like the Audio Control LC6 or others (like a 6XS) will take either a post or pre amp signal. I would not attempt to just wire a rca cable to the signal line between the head and the factory amp, definitely not a good idea. Look at the Audio Control LC6 + the bass remote for it. This will give you the capabilities that you require and provide a dedicated subwoofer level control at the same time.

www.audiocontrol.com