No sound from rear speakers

So, I was messing with my girls car this weekend and realized that there is no sound coming from the rear speakers. So I pulled the HU and the AC controls out of the way and I saw a cord, and from what I read when I searched I need the MITAH adapter. The back of the HU has two sets of RCA's. One for Aux in and the other does something that I have no idea about. LOL. So my question is if I buy the MITAH adapter will it solve the problem with there being no sound from the rear or is it possibly something else?

Thanks for the help!

Give us some more specs on the car.. Is it an infinity sound system.. LS? stock speakers?

The car is a 99 ES. It has stock speakers, the only thing that has been changed is the HU and that was done by the previous owner. From what I can tell it looks like a sop did it, because the install is pretty neat.

Just rip it all out, and rewire the sound... thats what I plan on doing with my GSX... But if you buy the adaptor and sound still doesnt come out, you have blown your speakers... The majority of people who drive around, drive with bad speakers, and it wont take long to completely kill your speakers. People drive around "bumping" to the music they like, and dont realize that the more they "bump" the more they decrease their speakers life. Speakers may say the have a 250 Watt MAX capacity, but thats max. Most speakers have up to a 50 watt RMS capacity. Even the speakers that say 250 watt, have up to maybe a 55 watt RMS. You dont want to run your speaker over the RMS very often, or at all. But people want to run their music at max volume... you do that a couple times, your speakers sound like shit, but most people dont realize when their speakers sound like shit... Woah, sorry about the ramble... it just irritates me when people think that have an awesome sound system, and it sounds like shit... Loud does not mean good.