Dead AUX input?? Help please.

[sstalder5]

One day a few weeks ago my AUX input stopped working. When I got out of the car it worked fine, and when I got back in it to go home the AUX input just made popping noises and didn't play music. CD and radio still work fine. Tried it with several phones/mp3 players and had the same issue. It's an older Alpine (CDM-9823) that uses the M-bus cable with RCA inputs for AUX. Tried a new RCA->Headphone cable, still the same issue. I'm hesitant to order a new M-bus cable though. Is it possble that channel is just dead? I can't find anyone with a similar problem and the guys at the local shop aren't too knowledgeble. (they're the "put two kickers in the trunk and it'll pound!!" type guys and tried to sell me a new one immediately) Have any of you guys experienced something like this? I know it's about time for a new HU but this one has been really good to me and I don't like much of the new stuff. Feel free to get technical, I'm not a noob to car audio. Am I missing something here? I feel like it could be a simple fix and I don't want to drop hundreds on a new HU just yet.