>> you need to add a 10K resistor between the mute line and 12V
> Can someone explain how a resistor gets installed? Does it just get installed inline?

Err, just do exactly what I said Install it between the mute line and 12V (which you can conveniently get from one of the power lines). Take care to insulate the exposed leads of the resistor to prevent shorting to other lines or bodywork.

Rob