You can tap the Green-Purple wire at the Black Bulkhead connector.
Printable View
I will second Toby's 3rd brake light kit, I have it installed on 0966 and I'm very satisfied. They light up instantly, much brighter than the stock brake lights. Plus when you're not braking the light strip seems to disappear, blending into the shadow of the louvres. If you want a 3rd brake light that is only visible when you're braking, his option hides wonderfully.
I don't have a better photo available off-hand but here's my D from behind. It's affixed under the top louvre but you can't tell it's there.
http://I.imgur.com/B5Z6T.jpg