Bypassing the amp stage seems rather easy, with a tiny integrated amp as the one I've ordered. But if you have a bluetooth phone things are getting (for me) more complicated. I can send you pics to your mail address.
A step-by-step diy, with photos, would be most useful for ordinary people, like me, who doesn't want to spend another $800 on an already expensive car, just because my favorite music only comes from the opposite side, since the driver side is mute and dead...