i saw one in action that was scanning for GPS, in the time it took to find a signal, i managed to put the postcode in my phone, my phone found it, i guided that person to the chinese which was 15 min away, went into the takeaway, collected and paid for the food and got back to the car and it only then just found a signal.
you couldn't have both GPS and MP3 run at the same time.
i've seen another where it had been spliced in to the cars harness, but no connectors were used, just the cable twisted on to a stripped wire, i don;t even think there was tape ontop.
I'm sure some people are happy with there's but from what i've seen and the price people paid, and how cheap all the parts are on ebay, i wouldn't be.