Behind the cluster would be the easiest and still hard to get to. I have the best spot for them where even an experienced installer would not find it within a 1/2 hour.
Having been an installer for years i picked up a few tricks, but being anal helps even more. Basically you want to hide the wires so that it can not be tpartyd back to the brain quickly. To do this right you would have to pull the harness and run all of the wire in the factory loom and tape it back up. Im sure there are very few people who have taken the time to do that.
On my B14 i had the harness out of the car and ran all of my wires in the loom and used actual factory wire. No splices were seen and nothing could be tpartyd.
I have some pointers. On a realistic note, put the brain up high or way out of the way and run your wires smartly so they are not seen easily. Take some time and take panels off to hide the brain. I have actually done a few in the drivers door with good success. The siren must be hidden, the best spot is in between the front fender and the chassis, NOT on the firewall. Use extra sensors wisely. Glass break is almost a must, (there are ways to break glass and not set them off though), a proximity sensor can be very helpful if set up properly, and a battery back up siren will help.
They are easy to install but not quick to install correctly. Also solder your connections, this will help hide them. Scotch locks or T taps can be spotted a mile away.