1/ get new headunit with 2 rca outs, NOW!
2/ run 4g to a fused distribution block. inline fuse within 30cm of the battery.
3 all gound wires should be grounded at the same point. u can use a non-fused distribution block backwards to achieve this the neatest possible way ( 2-1 instead of1-2 and one wire to the ground point)
4/ pick a very good ground point, i usualy undo a seat bolt and put it there.
mattius i do not know you but seriously WTF? learn some professional instalation techniques before quating to other people, especially about stuff that may burn down there car
and vector? thats is a very dodgy 4g cable u have there.
cheers and g-luck
P.S the jaycar power wire is very inexpensive cable that does the goods, much like the rest of their stuff. but u still get ewhat you pay for. i use stinger cableing and accessories exclusivly just because i want to. in a small setup or with no huge amps and multiple speakers there isn't a need to spend 400 to wire your car for audio. but maek sure you rca leads are good quality and that they are run down the opposite side of you car to the power cables,
cheers mitch