You could rearrange the question to find the answer, for example:
Problem........Citroen Saxo...
Joking aside... Both live and earth cable must be the same size. You must also use a common earth for all amplifiers. Amplifiers must be placed in a well ventilated part of the boot, and certainly not covered up. I recommend mounting the amp/amps on spacers, which will give it all round cooling abilities. The earth strap must be to bare metal, and well secured..
The amplifier may brag huge power, but if it has a signal to noise ratio of less that 90, bin it... and find one that has... I used to have a Harman Kardon amp that was quoted to be 22 watts per channel , with an s/n ratio of over 110.. It was equivalent to a 1000 watt pioneer at the time, and could comfortably run 5 speakers including a 15 inch Earthquake sub,which i changed after cracking my rear screen twice, having said that , the wiring cost nearly as much as the amplifier !!
Sound proofing is just as important .. If the car next to you can hear your music clearly when all your windows are up, then you are losing the sound which should stay in your motor.
Sub-woofers are designed to produce sub sonic sounds, between 30 and 500 hertz, your bro is right on that one. The speaker should run through a sub filter (on amp) , or an active or passive cross over network to achieve the desired level... I hope my info has helped..
