Low Level Input, JBL GX-A3001, 2018 Ford F-350

I have a JBL gx-a3001 that i am running one 12 inch sub-woofer at 2 ohms. I need to install it in my new 2018 F-350 and i am using the factory stereo. I want to use the line level input as the factory head unit does not have pre amps that i am aware. My question is, do i need to wire the left AND right speaker inputs or can it work with just one side being wired, sine i am using it as a mono amplifier. I'm going to use the rear speakers signal. Thanks

It is preferred to sum the L & R channels as input to the sub.

What if i am using a speaker level to RCA adapter instead of the amplifiers internal one. I have seen a post online that some people have just used one speaker side for their speaker level input to the converter.

Line out converter or RCA jacks, it is better to catch both channels. Although the information at the frequencies your sub will play, there is not a lot of stereo information in the recordings. You may never hear a difference if you catch only 1 channel.