I have an AVH-A205BT head unit.
I have not been able to get the 'ring' commands working, even if I send the PWM signals with the ring shorted to ground, it is just the same commands as it was with the ring open circuit.
I'm not particularly keen to keep trying, as the 4 commands on my steering wheel (vol +/-; track +/-) are in the primary command set.
Based on the circuits posted earlier in the thread, the ring needs to be pulled to ground to activate the second set of commands.
An optocoupler is not a bad idea, but if you drive the 'tip' direct with a PWM output you don't have the galvanic isolation that is the primary purpose of an optocoupler anyway, so I don't think it is much point in that case. If you wanted to fully isolate with both the tip and the ring, you would need to use a pair of NPN optocouplers between each input (tip and ring) to ground. Would probably also need a cap to ground on the tip input, you would drive the tip O/C with the PWM signal, and the ring O/C with the 'shift' signal. Personally I wouldn't bother, if you're powering your micro of the car battery, it's all going to be the same ground, so no real need for isolation.
If anyone gets the 'ring' commands working on the same unit as mine I will try again.
Kriszrap, wouldn't you need to make the 'ring' pin a digital output? My code is done with the Microchip IDE XC16 compiler on a PIC24FJ256GA702, not an arduino, so I don't know the correct syntax. Also, it looks like you are pulling it 'high' to activate rather than 'low'. My suggestion is to try shorting the ring to ground directly and sending your PWM commands again to see if you get a different set.
And cutoff freq = 1/(2 x pi x R x C)