Installing New HU in 97 A4 Audi -non_bose

Last time I did one of these I think what we did was bypass the amp, but we were able to use the factory speaker wiring (except for a few feet extra necessary to connect everything).  So the connection to the head unit was pretty straightforward.

For the rears, basically we used a DMM and a test speaker to figure out which wires were which for the plug going in to the amp,  as well as the plug leaving the amp.  Once we determined positive and negative input and output wires for the left and right channels, we left a few inches of slack for each plug, and then cut the wires so that they were no longer connected to the factory amp.  Then just hook input wires up to the corresponding output wires, using 16 or 18 gauge speaker wire to make it long enough.

I'd recommend cutting one input wire and then cutting the corresponding output wire, and immediately doing the connection, rather than cut all the inputs and outputs before even connecting any of them.  This way you don't have to label all the wires, and you also don't have to keep on testing and retesting to figure out which wire does what.