Chris,
You'll have to get behind the dash mate. Don't worry its a piece of cake. It's been off many times so will come apart easily. In fact you'll probably find most of the screws are missing so it will be even easier.
Follow this guide and you can't go wrong. Your just removing the center part so obviously stop when you can see the screws holding the stereo in and remove them to pull out the stereo. no need to take out your dials etc.;
http://www.internetwork.org.uk/kit_inst/dashrem.htm
According to the wiring diagram here: http://www.internetwork.org.uk/manual/system/radioamp/radioamp2.htm
The wire powering up the aerial on the car's harness is black with a red stripe so have a look for that.
I can't remember if there is a simple plug in 'ISO' type harness between the radio and the car harness, but if there is, the wire would normally blue.
There will be another blue wire kicking about at the back of the stereo that is the 'remote' for your Sub amp. It is possible that it's spliced into the same wire that feeds the aerial so make sure you fit the switch AFTER the amp remote wire or you will be swithing the sub on and off too.
Hope that helps.