Bose HU and aftermarket amp simultaneously wired

I have a 2001 Nissan Maxima with Bose HU and want to keep the factory unit and speakers. Yet there are no aux input options for the factory HU, so I am using a tape adapter now (sounds bad, but at least the wire is hidden). I am not sure if this is possible, but could I add an aftermarket amplifier and keep the existing setup?

To use the exising Bose speakers it appears as if a Scosche adapter is necesary (each factory speaker has its own amplifier).
http://www.crutchfield.com/S-SZEO6Gd...fo&i=142C4NN03

The adapter has four speaker input wires, could these be wired directly to an amplifier even though the adapter is designed to be used with a reciever (apparently has other wires that are supposed to be attached to the aftermarket reciever wiring harness).

More importantly could I have the adapter, which plugs into the factory wiring harness, connected alonside the factory HU. So that I could create a switch which switches the wiring harness input from the factory HU to the aftermarket amp. I know this sounds like a convoluted means of getting quality sound input to the factory setup, but I really dont want to get rid of the stock audio sys. Sorry this is so long, first post.

So to clarify, is there way I can hook up an input selector to the speaker inputs? I have seen these units for home audio systems, but would it work in a car?

Home Audio Amp and Speaker Input Selector
http://www.smarthome.com/82802.html