my advice would be to buy the correct lead as the 1/8" a tiny and the RCA coax wires are also tiny so soldering them togeother requires a fine pointed soldering iron, fine lead-based (rather than the often currently produced ) nickel solder, and some tiny heatshrink.
a shop bought rca-1/8"jack lead will be more reliable, and smaller, and less stress on the ipod's earphone connector, especially if you can find a lead that has a right angled 1/8" jack
instructions for stripping coax wire are at
http://www.benzworld.org/forums/r170-slk-class/1361561-upgrading-bose-premium-sound-system-2.html
as are the instructions for removing the two (heater outlet) panels either side on the centre console which ae located on the central sides of the footwells, this will give you a bit more hand room access
also there is how and where i fitted my overdrive
the two coax earths are twisted togeother and soldered to the longest terminal of the 1/8" jack, the smaller two inner wires go to the smaller two of the jack's terminals, and should be sleeved with small heatshrink ... rather than cutting the insulation around those two thin wires it is easier to run the soldering iron tip around the insualtion where you would have cut it, then pull gently, as it is easy to stress the copper strands and break them
from memory the tip of the jack is left audio, the middle connector on the jack's shaft is right audio, and the largest connector is ground
the gear lever bezel is retained by four tabs two at the front, two at the rear, put a piece of paper under the rear of bezel to feel their location, and pry up from the rear (of the car) .... similarly for the facia ... i will get full instructions from my home computer for you tomorrow
once that facia is off, and you have removed the centre console facia,(two screws exposed at the top of the HU cavity when HU and its mounting cage has been removed) then the four screws holding the ashtray will be visible. the ashtray needs to be twisted through 90 degrees to be removed and installed, and it has a small dash light wihich will undoubtly fall out when you pull it out.
with the gear shift facia removed you will see two biggish screws at the rear of the shifter console. the in-between-the-seats-rear-central-console/armrest is also retained at its rear by two nuts hidden under the carpet floor of the lockable central storage compartment, so lift up the rear of the carpet floor to expose them.
to remove the handbrake lever rubber i left a 60w lightbulb housed in an 'equipose" lamp near the rubber to warm it up.... after 5 minutes the rubber was warm enough to slip off easily, so no need to cut it off or blister your hands... soak it hot water for 5 minutes to put it back on
connectors for the cigarette lighter, roof switch, etc and air line for the storage compartment lock can the be pulled off, and you can instal your simpleroof opener
i cut a square hole in the bottom of the lockable centre storage compartment, and screwed a cut down plastic box it it to hold my ipod, so it is secured by locking the storage compartment, and hidden by the carpet, and surrounded by dacron to cushion it against bumpy roads ... four months later and no hard disk crashes ... but then my ipod is controlled by the HU, so i dont need to access it often
my ipod/HU adaptor also feeds power to the ipod to keep it charged
... my advice would be to use heatshrink and solder to make your splices, rather than crimps .... but that is just me, i tend to over engineer everything i do, but they never fail 
