If you want to use an aftermarket amp, your best off replacing your speakers, I don't think it's easily possible to use BOSE speakers as they usually run off of 4 ohms not 2. Speakers even better then the BOSE are relativly cheap.
You can't use an FM modulator without a head unit. If you use the BOSE amp and not the JBL one, you may be able to make it work with just a Ven-4 or other converter.
If you want to know what those connections are I would recommend you download the service manual for your car from http://www.nissan-techinfo.com/ . It will save you tons of time if you plan on doing any modifications to your car and it's only $20 for a 1 day pass.