I just helped my friend install an aftermarket JVC head unit with a wiring harness from Best Buy. Everything matched up and plugged in effortlessly, but when we went to turn the car on, he only got audio through the telephone speaker (hands-free speaker above the driver's door speaker).
What is the cause of this and why won't any of the other speakers work?
It is the Bose Concert system and we opened the amp to see if anything was messed up but it appears to be fine.
We have searched high and low, but can't find anything on the hands-free speaker. Maybe we are using the wrong search keywords, but we are getting literally nothing.
Any other information you need about the car, don't hesitate to ask.
Thank you for your help!

dont know about the phone speaker but check to see if there is a power wire for the amp.

+1 my thoughts exactly

We took a voltmeter to the amp harness and it was getting power.
As far as power wire for the amp, are you talking about the blue/white striped wire? Because we plugged that in too and still got nothing.
Neither of us heard of the phone speaker doing this before and it seems to be a rare thing. So rare that we can't find anything on it.

Not to threadjack or anything, but since we're talking about head units, does anyone know an easy way to get the rear speakers to work? With every wiring harness I've found you only get the front speakers. I've read somewhere that it's because the rear speakers are amplified, and sending an amplified signal from the head unit would blow the on-board amp. Anyone shed some more light on this?

Anybody?
Answers to SVT BRYAN's post would be nice too, as I am fearful that we will run into that problem once we fix the hands-free speaker problem...

Maybe take a picture or two of the wiring for the harness. It's possible you may have the wrong plug, but I'm pretty sure they only go in one way each.