It's actually very easy, especially in the Wrangler. All you need to buy is a 2 Channel RCA line out converter, example
, and tap it into your existing speaker wires. You can do this either behind the head unit, or as I did, tap the lines running into the sound bar near the base of the roll bar. Look up the wiring diagrams online, but if memory serves correct, it should be grey and grey white you need to tap. You can pick up a LOC at any car audio shop including Best Buy and shouldn't run you more than $20. You won't get a full digital signal, but if you are just hooking up a sub it doesn't make too much difference, especially if you are just talking about adding a little bit of bass.
You decide on which sub you want yet?