I drilled a hole under the backseat and right below the seat on either side of the truck is the frame. Really easy to get to. The frame of the Dodge Ram is not directly connected to the body and has Polyurethane bushings between all connection points to help lessen the stiff ride. These have to be replaced all the time, and trying to connect to the body will not give you a proper ground. When I get some sun out tomorrow I will get a picture of it up to you.
I like the larger option for speakers, but I hate the doors. SO many holes and very hard to seal for good mid bass response. I haven't heard the Infinity system, but I doubt it could be as good as a well thought out after market system.