Hi Ed. great project nice and cheap. One thing to consider. These PWM controls are low side switch devices meaning when power is turned low you can measure +12V on the outer shield of dew RCA connectors.
Touch this against you mount and you'll let the smoke out.
The issue arises with the use of RCA connectors as they have the shield unprotected (especially the
metal shield RCAs). The fix is to use a "high side PWM" switch controller (which has the shield as earth so it can only be earth and nothing but earth) or cover the RCA connectors with heatshrink. Discussion on this
here and
here. My old 6 chnannel design...i think I have changed it somewhat...have to look at it again. I'm using my 6channel device.
Although, to tell the truth, PWM dew control is probably not the best method. PWM puts spikes on power lines which can severely muck up delicate systems. They can also radiate RFI. I have not realy done anything on this but I did see one analogue opamp cct that had feedback and the output device was a transistor, low side sw. This could be made to high side sw design and is not PWM which negates the issues PWM has.
Lots of info on the web.