Originally Posted by Salc7
So how did GE3G do it and has anyone done that before
The way I described above. He just called his line output converter a crossover. A crossover is a filter that is used to separate highs and lows. Aftermarket component speakers have crossovers with 2 inputs ( + - ) and 4 outputs (tweeter + - and woofer + -)
So the wires that are running in to your speakers (a + and - ) contain what's considered a high level signal. A high level signal is capable of driving a speaker because it's all reddy been amplified by your factory radio. If you took that high level signal and attempted to amplify it it's going to start to clip and distort. So you use a line output converter to tap into your high level signal and get a low level signal that would not be able to power your speaker but your amp can then amplify it with out distorting it.
When using a line output converter you want to make sure your amplifier has a low pass filter on it because the signal you are grabbing will be a full range signal containing everything your speakers play and we only want the base to be amplified. So you set the low pass filter to around 60-80 hz and it will block all the higher frequency music and only play our base.
A aftermarket radio will have what are called preouts that you can plug your RCAs into and have the low pass filters built in so you don't have to bother with a line output converter.
Some amps give you the option to use high level inputs instead of low level so you don't need a line output converter but I recommend against going that rout as they are not terribly effective and can cause unwanted distortion.