even dead easy on the golfs...... rubber gromet at the back of the bulkhead...... feed it through done..... simple...........
i first started this stuff when i had my first car.....
found how to wire up headunits and then amps.......
wire from battery etc........ earths etc...... remote wire and rca's etc................
wheni wired my first amp up i put all th power and rca wires all scrunched up together...... and guess what it sounded brill no hiss!

which now looking back supprises me....
it was so good i had my locks done not long after but not to worry as my ''diy'' moss alarm sent them on there way...
empty handed

i then brought i book haynes in car entertainment manual...... brill tells you whats what..........
worth getting if your not electricly minded!

they also do this one aswell

bit of a max power book....... but thats all well and good if they show you how to wire up over the top installs!
but
off hand i know the following this is straight out of my head...........
black wire - earth
red wire - 12v positive
yellow wire - acc or igntion switch wire.....
on some stereos the yellow and red are sometimes
the other way around ...
orange wire - this connects to the lights this lights the stereo up with out it being switched on.......
blue wire - connect this to the remote imput on the amp or automatic powered airel!
brown wire - connects to mobile phone so when it rings it turns the stereo down.......
and the speaker wires normaly purple, white, grey, green, brown etc...... theses are not a problem the iso connectors clip together and all the speakers front back left and right are 99.9% of the time end up wired up exactly the same as the orignal stereo........ basicly the speaker connections are not normaly mess with the only thing to mess with is the power wires 50% of the time they can be wrong so the postive gets connected to the igntion ....... this shows up as either the stations cant be stored or you have to enter the code each time you use it........ ie kenwood mask.....
apart from this the rest is easy............
just dont ask me about........ protons and neutrons
reminds me of school days....... although we learned year after year about plants....... photo systhisis etc....
i soon started to ignore plant projects....wonder why?

and most importantly like he says dont pay anyone to do it .do it yourself.......... unless you havent got the time....