Not to step on your toes, but Ive been in the car audio business for over 15 yrs. I have installed for 10+ of those. Worked on many vehicles much more complex then the bmw systems.
I never said to "chop" any speaker leads either, I said cut the end off of a good twisted pair rca cable, next you would then unplug the input/ output to the amp (now the amp is dead and out of the loop), solder to the proper leads, then feed the aftermarket amp, finally from the aftermarket amp to the speakers in the car. (This is only one of many easier options) It is possible to use the factory output wiring at the amp as well for 25-50 watts. It really isnt necessary to rewire the whole car, bmw does a good job running twisted pair wires everywhere to negate noise. They dont need to be shielded either, some of the most expensive/high quality audiophile cabling is just twisted pairs with no shield. That has been proven to be the best setup time and time again in the car audio environment. Ever open up a pair of high end Monster/Rockford/PG/LighteningAudio/Directed electronics/ Esoteric Audio cables? I have.
BTW, most reputable amps can handle at least 8 volt input through their rca input, some even 15 or more. One brand we sell, Zapco is quite flexable with their Symbilink system.
I doubt he is going to do this himself, but hopefully with a little knowledge under his belt he will understand all of his possibilities rather then just doing what the shop that blew up his factory deck wants to do. We try to educate the customer about what their factory system consists of and what we can do to make it better. There isnt too much out there that cant be worked around. There are many great companies that make "little black boxes" to interface with all kinds of good stuff.
Ive had good luck with L.O.C.'s, as long as it is a quality piece and installed properly they work just fine. Dont be scared of them, be scared of improper install.
The hk piece is a very high quality am/fm tuner with cd that is also a navigation unit. Crutchfield is blowing them out cheap right now... Its worth a look definatly.
Best regards all,