It seems you have learned a lesson from me when I hosted JOC4B - this should help clear up any confusion.
Correction: the rear seats must be full-sized, not +3.
The third brake light can be mounted at the top of the rear fascia or in the interior, just ahead of the rear window.
It is possible, but not necessary, to have the car meet WES 9 or later - but considering the era in which this challenge is set, a 3-way catalytic converter of some sort will be required.