It's for the left side of the console. I too have one last blank :-) That;s why I bought this. But ended up with a different idea. Even the two blanks in the center console have been replaced by a garage door switch and a switch that turns on the various extra led lights I installed last year...
I was expecting that it would connect to the fuse panel (or somewhere near the spoiler switch). I have read the instructions and it looks like it connects to the gray harness (or however is it called) behind the instrument cluster.
I plan on changing the led lights in my Boxster (one more time, I think it's the third time now

), so I did not go through the instructions thoroughly because I am too bored to remove the gauge again.
I'll scan the instructions tomorrow and I'll post them here. Despite the fact that is not OEM it can be very useful to be able to see when the spoiler goes up. Once I had my wife standing while I was driving at 90, just to check that the spoiler works as expected (well, of course it does

).