Just to put this into perspective, Bill doesn't owe you jack s&*t. He is not a vendor, he is one person with a hack..er excuse me, home made modification that a few people bought into. You are on your own here just the same if you bought some sound system off some random guy on Craigslist. There is a trade off here, sure you got your car going for much less than fixing it right....Now maybe you are seeing that there is a downside of these kinds of mods.
Now just to clarify further, I don't know Bill personally, but I'm sure he will not leave you high and dry. He just doesn't strike me as that kind of guy.