hi
I recently did this in a mk6 golf and to be honest is so simple with a audio line out converter, which you simply splice into the the rear speaker wires there are four wires
left rear negative
left rear positive
right rear negative
right rear positive
simply cut this in half solder in the connections from audio line converter, mine came with a remote and earth and power cable as well, so then you simply splice in to power cable and earth on back off stero solder up connection plug in your rca done obviously you have to run power cable from battery obviously but it is very simply job whilst keeping completely factory look
