I dont think electrics is anyones best subject...
Found this anyway from when Adam R done his amp install
guide im sure he used the switched live from the back of the HU for the remote wire for the sub...Not 100% on this though
6) if you are going to wire up the REM to the HU then read this, if not skip to step 7.
undo the 2 screws under neath the HU (left and right side at the front just underneath the facia)
the HU faci should then slowly prize off.
carefull at this point as there is a ribbon cable attached from the facia to the HU behind. detach this and put the facia to one side. (i have not removed the HU but i will give you the best account i can out of memory from other cars) undo maybe 2 screws and then the bottom part of the HU should pull out, you may need some HU pins to pull it out.
once this is done there will be a serise of conectors at the back on the bottom left hand side. the black ISO connector sould have numbers on it in small print, if not then look for the ONLY BLACK WIRE. pair in to this by splicing or soldering. attach the REM cable to the BLACK WIRE then feed it around to the passanger foot well.
put everything back to how it was (regarding the HU and facia)
7)
this way ONLY TURNS ON THE AMP WHEN THE IGNITION IS ON.!
other way of doing it is a bit more of a guessing game, get some one to turn the ignition on and off while you hold a multi metre over an earth and another colour. when ignition is off you want to see 0v, but when turned on you want to be seeing 12v. (will be around 12.4v)
from this cable pair off and this will feed your REM.
This is the full thread with pictures aswell if this helps.