hardwired

Does anyone have any experience with these? They're the devices that allow you to connect your MP3 player/phone/laptop/DVD player to your car's radio. Similar to an FM Transmitter, but instead of transmitting, it directly wires into the aerial cable on the back of the radio. I'm considering...

I'd like to have hard-wired power supplies for satnav (Garmin) and for a [forward looking] camera, as I hate wires running around the dashboard. I bought the actual sets, but am not quite sure if I should attempt to do it myself, and do not know anyone locally who I can trust to do it for me...