The best advice I have is to replace all of the Bose with aftermarket. I know that's expensive, so your idea is a second choice.If you have an aftermarket sub, you MUST get an amp. There is not going to be anywhere near enough power on anything that comes stock in your car to run a sub.
I am assuming you have an 03 Max, so I am not sure if you can use the 95-02 interface most people use. If this doesn't work for your car, you might have to do more research. PAC probably makes an adapter for it, but I am not sure.
Here is the 95-02 one:
95-02 Bose interface
It converts your Bose wiring to add additional Front and Rear RCAs, along with a remote lead, so you can install an amp, thus you can put in a sub or other speakers.
