I want to bypass my headunit and connect my mp3 player directly to the amp
and to the sub amp. I am going to run one RCA cable to both (headunit runs
2 separate RCAs). In order to turn them on I either have to turn on the
headunit (at zero volume) or connect 12V to the blue wire coming out of the
headunit. Will I damage anything if I use a switch to turn on/off the amps
with this blue wire?

TIA,
Leon