1: What is the Year, Make, and Model of the vehicle.
2: What is your budget?
3: What exactly do you want your outcome to be? (Good sounding music, or loud bass?)
4: And specific sub sizes that you prefer more than others?
5: How much trunk space do you want to use up?
6: What type of features are you looking for in a CD Player (aka Head Unit)? (Ex: USB, AUX, BlueTooth, etc)
7: What type of music do you listen to?
8: Is your stock stereo system amplified? (You may or may not know this)
9: Do you have Steering Wheel Controls? (Volume/Seek on the steering wheel)