Installing amp + sub in trunk. Can I get line out from back speakers?

I read the write up on how to remove the stereo to tap into the wiring for the speakers so you can get a line out converter working.

Since I have to find a different place for the remote, and I'm getting 12V straight from the battery, do I need to take out the stereo?

Couldnt I just get wire the line out converter to one of the back speakers? Sounds like a whole lot easier time this way.