Hi Gizmo,
I had the same problem a while back and got some good advice from the rest of the guys here,
this thread outlines roughly one way of doing it. The short version is (Autocom will tell you this as well if you speak to them) carefully peel back the boot over the rear brake pedal switch - take your time so you don't rip anything. With a multimeter find which of the two connectors is live (you don't want the Autocom to work only when you're breaking after all....), then splice in a live feed lead, remembering to pull this through the abovementioned boot. Secure with either a solder and/or self amalgamating tape (fantastic stuff). Carefully put the rubber boot back on (this took me 20 mins, don't lose your temper with it) and run the lead up to where the Autocom is, tidying cables all the way.
You now have a switched, fused live for the Autocom, take earth straight from the battery.
If you want to go further than this (and you're a bit paranoid like me), you could also put in an inline automotive fuse on the live feed to protect the Autocom itself (5A should do). This will also allow you to just pull the fuse if you want the Autocom off for a while.
Another step further is you take the abovementioned live feed with inline fuse and feed this into a connector box, where you can take off power to other things, e.g. GPS, radio etc.
You can see a couple of ways people have done this in the Comms and Electrics album,
this
was my end result (sorry about bad picture), works a treat...
