That one speaker cable is probably useless. If it were two cables, that might be different. You need two cables for rear left and right speakers. But, maybe its not a speaker cable but the interior light cable. It should be easy to check if that cable has power with a electrical light probe(I think that is what its called). Also, it could be for a subwoofer setup. I would think it might be fairly easy to tap into your door lights, and into the stereo for rear left and right, from behind your radio, and run those wires through your roll bar. I would push a roll of that wire cord through the top of the roll bar until it comes out the bottom, and then pull your door light power and stereo wires back through to the top of the roll bar. I think the wiring goes through the roll bar.
As for the Sound bar itself there are many options. I would prefer to go OEM myself. This is because none of the speakers, sold with aftermarket soundbars are going to be that great. The OEM soundbars have 5.25 speaker setup, and you could match that size with your front speakers. Get four 5.25 speakers. Two for the front, and two for the rear. Some people prefer Polk(45RMS), some prefer Infinity Kappa(55RMS), but I prefer CDT audio speakers(60-110RMS) from California.
It is definitely a mod to consider if a good sound system is of interest to you.
Rugged Ridge has a aftermarket soundbar that looks great, but everyone says the speakers just aren't that good. So, you will probably end up switching them out, and the added cost is ridiculous in my opinion($250 plus new speakers). Plus, those speakers are 6.5, and it would be hard to match the same size in the front. The front speakers are 4x6, and they suck at that small size. Everyone prefers an adapter for 5.25 in the front. This is the way I went after blowing out the weak 4x6's in the front, although mine were great aftermarket 4x6 Infinity speakers. 4x6 is just too small. Different size and/or different manufacturers of speakers is going to compromise your full balanced sound.
This is why I wound prefer to get a new or used OEM soundbar, with the 5.25 speaker holes already setup, and then match them in the front. You will get awesome balanced sound this way.
Link for Rugged Ridge soundbar which looks great. 6.5" speaker setup with 2 or 6 speakers.
Rugged Ridge Jeep Sound Bar - Jeep Wrangler Sound Bar Speakers by Rugged Ridge
Used OEM soundbar on Ebay. I don't really like it because they say one side is faded. But, you could probably spray it with a vinyl black spray that should adhere well to the material if cleaned well. It only cost $100 dollars. It says it has the harness but, you will also need the hardware to attach it. I would contact the seller to find out if it comes with the harness and the hardware. Good luck, Hope this helps.
Jeep Wrangler TJ Overhead Sound Bar Speaker Soundbar 97 02 Speakers STK P | eBay