And I'll give you some free legal (I'm not a lawyer but I did stay at a Holiday Inn)
Read This
http://www.sema.org/main/semaorghome.aspx?ID=50096
Then also understand that most companies never said "you can't" so when you do it doesn't make it wrong.
If putting an amp in a car (one can argue they offer a OEM radio upgrade and whats the difference in aftermarket) is going to "blow up the car" then it should be explained in the beginning.
If we believed all the BS dealers give us (honestly most people who work there don't even own a BMW or are into cars) I couldn't change to non-RFTs, but different wheels on, change the radio, change my own oil...and the list goes on.
They do it to BULLY and protect profit...despite the fact that lots of times it's not legal to do what they do.
Educate yourself, and that may require some "legal" help. Spending a few bucks might help you when the next thing goes wrong and they blame it on your BAZOOKA tube.