It sounds like the prior owner was either lazy or didn't know what he was doing. I believe in doing things myself and the right way.
The Jeep's dash is very easy to fish wires through to the front speakers. If it was me, I'd go grab a spool of speaker wire from RadioShack, BestBuy, etc and just rewire everything. It might end up being easier than trying to figure out someone else's mess.
To do this is easy and not difficult. Download the owners manual for the head unit and it will have a wiring diagram showing what wire go to which speakers. I'd just run fresh wire (3' each) to the front speakers and then also run fresh wires to your soundbar. It's really not difficult or very time consuming.
As far as the amp. Most likely it will have a set of RCA (red/white) jacks. You can run a set of RCA wires directly from the back of the headunit to the amp. That will provide sound frequencies to the amp. There's only 3 other wires you'll need to hook up. A heavy guage (fused) wire to the battery, a short ground, and a remote 'turn on' lead that can also be run to the back of the head unit.
Downloading the owners manual will tell you exactly which wires (color coded) from the back of the headunit go where.
Just my .02, but I've been doing this a while and am pretty comfortable with it.
When my time comes, mix my ashes with gunpowder encased in brass, point me towards murderer's and pedophile's and let me fly.
(: ˇ ɹǝʌo ǝɯ 11oɹ ...sıɥʇ pɐǝɹ uɐɔ noʎ ɟı