Most Bonnevilles with the permium sound system do have an amplifier but it's only function is to boost the bass to the woofer section of the rear speakers. The radio's internal amp is independent and you should still hear sound and be able to control the volume. The CD player in my 93 has a broken volume switch and although I can not adjust the volume, I can still hear music.
You can try
www.the12volt.com. You'll need to subscribe (it's free) and click on Vehicle Information under the General Information heading on the left side. This information usually applies to the installation of an aftermarket radio but it might help you.
For schematics and factory service manual information, I subscribe to
www.alldatadiy.com. This will cost $24.95 for a one year subscription to the vehicle of your choice. Not a very cost effective solution if it will only be used to correct your radio problem but if you intend on repairing the vehicle yourself then it's nearly as effective as buying the actual factory manual at
www.helminc.com
Good luck and keep us up to date.