So, here's a summary...
1. Class D amps (for efficiency and less power consumption)
2. Marine speakers and other equipment (for water resistance)
3. Match your enclosure with the speaker (consult a pro)
4. Consider power requirements
5. Use quality wiring kits (consult a pro)
Note: "Pro" does not mean back yard or small time installers. A lot of small time installers/sellers will deliberately wire your system to make it sound mediocre and will talk you into buying additional equipment and spend more. Just a friendly warning..
Just to share our show car .....
I used Pyle in this ford focus, lots of em'... too many to list... I can list if you want...
Lanzar in the BMW X5....