Don't connect the amp to the battery negative terminal. Find a factory grounding spot near where you mount the amp and use that instead. Shorten that run of noise gathering cable.

Your JL amp is stable down to 1.5 ohms impedance (it's not resistance, similar but different). Run that sub in a dual 2 ohm configuration (which according to your link that sub does). Lower ohms equal higher watts to the speaker. If you go too low, you fry the amp but that's not a problem here with your single sub.

There are a number of hookup options, but in all of them you are going to have to take the signal wires and convert them to RCA male plugs, likely via a line out controller. BSW do some more searching here and you'll find some threads on subwoofer installs. I haven't done this, but will be via a much more involved process in the future.

Good luck!

__________________
Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual