I'm also interested in adding a music system. I know, I know - exhaust note, pipes, music to your ears, REal Cobras didn't have music etc, etc, etc. Guess what - mine isn't REal and I've heard this song and dance for years so save yourself the trouble of repeating.
My car is loud but not that loud at slow cruise that I wouldn't be enjoy some tunes from a good quality installation; plus I'm not always driving when I'm out with the Cobra - sometimes it's just parked at the beach.
I'm sure others have done it - like this Backdraft from the for sale section. Love to hear your thoughts/ideas/experiences.