in all honesty car audio is a dying art. with manufacturers equipping cars with pretty much everything it is getting harder and harder to make money. Only true audiophiles still have a passion for it and most of them either have the knowledge on how to do it themselves or take it to a custom shop. Alot of the jobs now are consisting of integration with the factory components and that can also be a little difficult. another thing is alot of cars (Volvo, Saab, Etc...) are starting to incorporate a kind of data wire system where a bundle of wires are a thing of the past. They pretty much have a couple of wires or even one that will control multiple components and the only way to differentiate between each is a certain resistance. Also some manufacturers are, soon, going to be going from 12 volt systems to 24 volt systems which is a whole new realm along with electric cars and etc.
That being said you have to make sure that it is trully something that you want to do. If you were to try and land something at a retail store (best buy) you may not get paid alot. While if you are trying to go to a custom audio shop you may not get paid alot as well. What it all boils down to is how fast you can learn and comprehend and you experience. The real money right now is in custom work other then that it is pretty much dying out.
Im not trying to deter your way but I just want you to be informed on what you plan on getting into. Being familiar with electronics is one thing but when dealing with car electronics its another thing. I had a guy that used to work for me that was an ET (Electronics Technician) in the Navy for about 10 years and had the understanding but couldnt grasp the concepts that we were doing and could not keep up so he resigned.
If you have anything specific you can ask me and I can let you know with whatever you want to know.