Yeah, I'd have to kindly disagree with them. The riocar and BOSE amp are both at line level (and match pretty nicely).

The parts I think you want are:
1)RAD-RCA
2)OEM-1 box
3)AMP-GM21 or AMP-GM32 depending on 12 or 32 pin harness. That's the way I did it. (Except I needed a different AMP- cable)