Beginner Amp Bypass Question(s)

[Questos]

So, I have this problem. GF has a 05 vibe w M&T and I want to put an amp and new speakers in it but keep the factory HU. Problem is, I have no access to the car during the week b/c we don't live in the same place.So, since the car isn't mine, my familiarity is minimal (but getting better). Please bear with me.First question:Obviously, I'm not going to splice into the factory harness so I need a Metra bypass harness. For the 05 model year, the only applicable harness that comes up seems to ignore the additional 8-pin connector in her factory setup. I can't seem to find it individually listed, either. The 70-2056 seems to be for model years up to 2004. What's the deal?Second question:Does the factory harness below the passenger seat (leading out of the wire cluster and into the M&T amp) match the (2) plug(s) behind the HU? Seems like it would be 1000% easier to add a bypass harness at this point in the system versus rewiring all the way from the HU.Final question:How is the factory amp attached to the floor of the vehicle? The brief look I took most recently showed what appeared to be two arms running over it, suspending it off the floor. Do these bolt into the floor on either side and screw into the amp? Can they possibly be re-purposed to hold an aftermarket amp?I plan to rewire all the speakers after the amplifier, so I am not concerned with anything after this point in the stock system.Thanks in advance for any help you can provide. The more I know going in, the more of my weekends I get to spend w my lady instead of rifling around in her car.