From my research, yes you can splice rca leads into the 20 pin din cable - only use rca output, do not wire regular speaker output from your head unit to your amp this will likely damage electronics as it is designed to power the speakers, not just send an audio signal.
I just purchased a kenwood ddx7017bt and will be installing it in my 2006 MY07 3.0r spec.b this week and i will be keeping the stock mcintosh amp. assuming all things go well i will be attempting to take many photos and post a thorough guide on this forum as i dont feel there is a detailed enough guide and it could be beneficial to the community. so waiting for that may work in your favour. Im hoping to have the guide up next weekend assuming i find the spare time.
in the mean time, take a look at gfennessey's post (
viewtopic.php?f=22&t=12400). his post has a lot of information and what i will be doing is largely based off his work.
however, gfennessey's post doesn't cover things like the self learning swc that kenwood (and other brand) units have. i will be installing these and it will be included in my guide