You do not need (or want) capacitors in the circuit as they act as highpass filters and will degrade the sound. A 300K 1/4 or 1/8 watt resistor across the left and right signals (white and yellow in your case) is all that is required.
If you have an uncontrollable urge to add caps then make sure the resistor is on the head unit side of things
If you have an uncontrollable urge to add caps then make sure the resistor is on the head unit side of things
