What year car?

I've never seen shots of having to install the actual harness routing, though there's plenty of info on installing car PC harness, and that is where I would start, if I wanted to route cables..

for a pre 03/2002 = BigPatsFans.com
for a post 03/2002= BMW Bluetooth installation instructions for e46 3 series

Don't know if this is at all helpful though..