I've decided to use a 4pdt relay to switch between my car's head unit, and my car computer output into the factory JBL Amp. When testing things out I used an ipod in place of the car computer, and everything worked out perfectly.
Here are full details of the project http://www.kusnetz.net/prius/
I hooked up my computer, and I heard some nasty loud pops, a high pitch squeel, and then I smelled burning electronics and there was some smoke. I unplugged everything. I think I've partially damaged the computer's line out as when I just hook up headphones to it the sounds seems to be missing some mid-range frequencies.
I am using this as the pre-amp PIE LD1 2-Channel adjustable Line Driver
Anyway, I think I may be getting voltage coming out of phono jack that is connected to the input of the preamp.
Touching the tip of the phono jack to the ground of the car I'm reading 6.5 volts. I'm guessing passing that voltage through the computer's output is not a good thing. I have no idea where the voltage may be coming from.
I've wired the preamp to the wiring harness of the JBL amp. I'm using the 12+v and the wire labled amplifier ground as the ground.
Any ideas on what is happening? I'm guessing why things work fine with the iPod is the iPod isn't grounded anywhere, while the computer is grounded.
Also I can bypass the preamp, and just plug the computer directly into the relay and everything works fine, although I need turn the radio all the way up to be able to hear the sound.