A:AnswerConsult your local Best Buy's installation bay. Some auto techs do not wish to do half of the job. Some are willing to. It depends on each individual tech. At minimum you are probably looking at $50. The amp install is $120 for subwoofer amplifier if that's what you are installing and it comes with a lifetime labor warranty.
A:Answer3" deck screws, just kidding. There are various options to get your subwoofer from becoming a projectile. Speak directly with an auto tech to come up with a solution that satisfies you.
A:AnswerWhich sub?? Typically a subwoofer requires more power to play than a set of full range speakers so an amplifier is needed. It isn't necessary that you have an amplifier but you can purchase one at time of installation. The auto tech can help you with that process.
A:AnswerI would contact your local Geek Squad installer ... it is a guaranteed good job! I used to do all of this type of work myself but the value and quality of work makes it so that I just let my local guy do it! Good luck!