sub to stock head untit

Is it a stock head unit from Nissan that came with the truck or is it an aftermarket unit from Rockford Fosgate?

If it's an aftermarket unit, there should be a model name/number on the faceplate somewhere. If you can get that, we can find out if you have sub outs or not. An aftermarket unit usually does so it should be straight forward.

If it's a factory unit, that's going to be a bit more difficult and you'll need something like this:

https://www.crutchfield.com/p_161LC2IB/AudioControl-LC2i-Black.html?tp=61671

Or at least a sub amp that handles high level inputs. That way you can take the speaker leads from your rear speakers and connect them to the high level inputs on an amp or an add-on subwoofer crossover and then use that to send a signal to your amplifier for your sub.