Stereo Receiver Problems
 

Hey everyone!
I'm new to the forums, and as much as I would like to do an introductory post first, I feel like this is a bit more important.
I have an '06 Shinka that I picked up a little over two months ago. About a month ago I noticed that the stereo system was not working as intended. All of the buttons on the stereo, as well as the air direction selection button do not work unless the rear defroster is on. At first when I discovered this, I would turn the rear defroster on and about 15-20 minutes later it would turn itself off and the stereo would just default to the last AM/FM selection I was on (if I was listening to CDs) or to the next selection in the round (AM, FM1, FM2).
Recently though, the stereo will go completely haywire and start toggling the rear defroster at a bit of a fast pace completely at random, and when it stops, the rear defroster is completely stuck. I have full use of the rest of the buttons, sans the "air direction selection," and if I so choose to listen to CDs, it doesn't allow me to switch back to am/fm.
Here's what I know about the car that might be relevant: After taking the car into the shop for a CEL and to replace some of the bulbs on the side, I was told that it wasn't the bulbs, but the wiring itself. They told me that it looked like rats or some other animal had lived inside the car and chewed up most of the wiring. They said that they had fixed the wiring, and the problem didn't arise until a couple weeks after the check-up.
So here's my question to the (hopefully) car-savvier community: Would it be worth it to take it in to get it looked into further, or would it be a smarter investment to get an aftermarket stereo if possible? And, assuming I take it in, would a mechanic be the place to go, or somewhere that specializes more with stereos and the likes?
I'm not too car-savvy, though I really do want to learn, and I appreciate any feedback from the community!
Thanks!