Hi,
I wanted to do the same thing, but after removing the single DIN unit from Spec S, I found out that my wiring harness did not have the plugs for the Spec R double DIN unit. Nissan Australia does not sell converter patch cables that will allow you to install the Spec R double DIN unit into the Spec S.
In case that you want to fit an aftermarket car stereo unit, read on!
Now, if you want to remove the centre console and the single DIN trimming, it is quite straight forward.
Get you hands on the S15 Electronic Service Manual from Nissan.
I think that it's source has already been posted on SilviaNSW.
Body Trim chapter of the manual shows how to remove the Instrument Panel Assembly, which includes the Radio surround.
A word of warning! Silvia interior trim is easily marked and is easy to break! Buy a plastic cooking spatula from the Go Lo or Reject Shop and use it to pry loose the gearstick surround panel. Once the panel is freed, use the Phillips head screw driver to free the interlocking panels underneath and work your way towards the single DIN unit.
Cheers!
