The line level converter should solve your problem, and it's a pretty standard Scosche part. If the Bose in a 300ZX is anything like the system in my Infiniti Q45 which also has a full Bose speaker setup, you want this:
Scosche SLC-4
I don't think you'll need anything much more specific than that, but you will definitely want to pay attention to what the earlier post said about wiring in the head unit. The 300ZX harness can be kind of tricky, and doesn't work well with some systems even after all the Bose gear has been ripped out unless you make sure to wire in that power on line. Search the forums for info on that.