New here and looking for advice.

I'm looking to install a lead from the back of the radio to allow me to connect an mp3 player, but want to keep it as discreet as possible. Ideally, it would be good to route to the glovebox behind the handbrake to utilise the power there for charging but the passenger side glove box would do.

It's a non standard JVC head unit so I know I need the KS-U57 adapter to get me started, but wondered if anyone had been successful in doing something like this? How easy is this to do - or am I better upgrading the head unit to a newer model with an aux in on the front?