Are you sure the harness going to the stock amp does not have a remote turn on? Do you have the cd changer in the trunk, if so check that harness?

 

The problem with that idea is that both the amp & cd changerare hooked up opticaly.. I plan to tap into a fuse as the S-type has a fuse box in the boot of the trunk. It appears that I have no other option?

 

Let me try and help...do you have any labeling for the fuse box in the boot? It doesn't really matter what you run the fuse to just make sure whatever your connecting it to turns on when you turn the ignition on.

If you want to do it and get it over with just run a remote wire from the from the front fuse box to the amp you are trying to connect. That way you don't have to worry about chasing down a remote wire with the factory amps nor worry about grounding it out and mistakenly messing up something you didn't want to.