I had a Bose system in my 97 Maxima, ripped it all out and installed aftermarket (long before I decided to build and install my carpc), but in the process of decision-making I learned about how the Bose system works. The HU sends out a very low power line level signal to each speaker, which are individually amped at the speaker. You need a line level converter (Scosche makes one, I saw it at Walmart) to attenuate the output from an aftermarket HU to the Bose speaker amp inputs. It can be done, but it doesn't sound nearly as good as full aftermarket system.
amplifier separate from the HU, so the HU is just sending out a line level signal. I switched out the HU with an aftermarket one and had to buy a Scosch line level converter at walmart for $20, because the aftermarket HU has a built in amplifier (so I needed to convert this amplified signal back to a line level one). So, if I'm not wrong, since the signal coming out of the computer isn't amplifier, you can just connect the line out from the computer directly into the lines provided when you buy a radio wiring harness for your car without needing the line level converter. But I could be wrong; the line level converter I brought has 3 output options (can't remember off my head since I installed it half a year ago) and maybe 1 of the 3 output might be needed. I will be installing a carputer in the same setup as yours within a week or two. Already ordered the parts few days ago, just waiting for the items to come in. I'll keep you posted.
The "line level" is different from the Bose than normal, I believe the Bose is higher level. Read here:
And for even more info on Bose systems, look here: