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PWM_OUT ----////----+------ TIP
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R is 180R, C is 47uF for my circuit, I would think anything from 1u up to about 220u would do the job, too much lower you will get the ripple through, too much higher will affect the rise time. Don't change R too much. Too low there you will have drive current issues, too high and it won't pull the voltage low enough for the lower resistance commands.
You will probably have to experiment with the peak voltage and PWM values. Measure the voltage between tip and ground with no resistor. It is 5V on older units (apparently), 3V3 on mine.
I still haven't got the ring working, not likely to keep trying as the commands on the steering wheel (track up/down; vol up/down) are all there.
And for what it is worth, my Volvo XC70 CanBus address for the SWC is 01 31 72 6C when read by an ELM327. The bits in the last nibble of the last byte each correspond to one of the switches.