passive crossover question

using a factory system that is internally crossed over at the headunit at lets say 150hz down for the midbass door speakers. what value capacitor or resistor would i use to get the speaker to play at the lowest 60hz ? this way the speaker isnt playing lower then it need be, elliminating distortion as there is a sub in the system to take up the slack.

You would have to know first what the nominal impedance of the factory speakers are.  If they are 4 ohms, you could put a cap of 400 uF on the +speaker lead before the speaker.  Here is a calculator.  And here is a cap that would be good enough to do this.  If the speakers are of a lower impedance, use the calculator to figure what cap to use.  I used 100 Hz as the starting point for a 6db/octave filter for the example.

hey thats great! i would like to keep it at 65-70hz up so with the speakers being 8ohm im looking at a 330uf cap for each. thanks for the great information!