If your car stereo was any good I would get that too. You can order a replacement faceplate and installation cage if needed, install tools, whatever. I have done it before, pulled nice stereos from crappy cars with none of the hardware I needed. The parts are ussually available online.
Im really surprised you are having such a hard time. Why would they charge you money for something that is yours already? You haven't been compensated for the car yet. That ammounts to someone stealing your stuff then selling it back to you. They should give you back anything off the car that you want, then base the compensation payment on whats left, not the other way around. Also, its not up to the salvage yard, they are just holding on to the car until the insurance company tells them what to do with it. Its not thiers, its yours. And before it can ever be thiers, you need to first transfer ownership of the car to the insurance company. They can't sell you back your own stuff when you didn't release ownership. Its not thiers to sell, that would be stealing. If the insurance company sold the car, without you selling it to them or signing off on it, that's also stealing. They are defrauding you, don't take thier crap. Call anyone with authority that will listen and plead your case. Nobody will fight for you if you don't make your situation known. Also, don't forget, the reason they are giving you a hard time is because its an impreza, and just because its wrecked, doesn't mean that its worthless. They are doing this becuase they think they can get free stuff out of it. They want everything that is in the car that you don't take because they can sell it all on ebay and improve their profits. That includes your mudflaps and your stereo and anything else that doesn't come on the car standard. If you don't fight for it then you don't get it back or get compensated and they make a larger profit. The fact of the matter is, if you don't bring it up with the insurance company, they wont cover the extras like the flaps and stereo. It would be like you never bought them, aside from the hole it left in your wallet. They would only see the cars blue book value, and compensate you based on that. Sorry, just hate to see people get scr*wed over, this concludes this rant.