First thing to look at is the factory speakers. OEM speakers typically are paper composite dual cone – anything but special. A relatively cheap option for upgrading is to install Coaxial speakers (tweeter and mid-woofer in one, easily installed, unit)
^ a typical coax (ial) speaker
The next limiting factor is power from the factory headunit (CD player / Radio). Now the simplest and cheapest way of doing this is to replace the head unit with a single din size CD player with a higher MAX power output (4 x 45watts MAX for example). The problem with this solution is that a.) not everyone wants to change the stock CD player out and b.) they are cheap for a reason, cheap components cheap build quality and they quote MAX figures because their real world RMS figures are more like 15-17 watts RMS.
So one way of keeping the stock unit and adding an external amplifier(s) is to use a LOC (line output converter) basically what this does is it converts the signal normally sent directly to the speakers to a signal that can be used by an external amplifer. Simple eh!
^ a simple 2 channel LOC
Ahh but there is a catch – external amplifiers need to be turned on some how – the solution is to either make the amplifiers turn on with the ignition regardless of wether the radio is on or not, this works but can make un desireable system noise as the amplifier runs with no input signal or you could use this little device from PAC (distributed in Australia by Stinger Electronics – most GOOD car audio stores will be able to get this in for you if they don’t have one in stock) its called a TrunkLOC
^PAC TrunkLOC
The beauty of this device is that it is a LOC and a low signal relay with 2 second delay. This stops amplifier “POP” (this can be caused by the amplifier coming on before the signal). It supplys 12V (@ 2 amps max) signal to your amplifiers “remote” terminal
Now I wanted to amplify my fronts and add a subwoofer so I used a simple LOC for my fronts and a TrunkLOC for my sub (running off my rear speaker wires) this way I keep my rears running off the head unit, my fronts are amplified and my sub has signal too. But sometimes you want to turn the sub down – or occasionally (read mostly) you want to turn it up. This little device (also from PAC (LC-1 is an RCA volume control or LC-2 same as LC-1 but im lead to believe this has a built in standard LOC but Jaycar have a similar item but I don’t recommend theres as I’ve never managed to get it to work properly) become a “volume” control for the sub relative to the volume of the head unit.
I hope this little write up helps someone out there.
*Note I am in no way affiliated with PAC or Stinger Australia. I am just a very satisfied customer and I recommend products I have had good experience with