On some pictures of RX-8s I can see there are small square "plates"
under each head light. (Not on mine).
What are those?

Magnus

 

in europe they get headlight washers that pop up. it isn't available here in the US or in Japan I believe.

 

Those little squares are for headlight washers or whatever they're called... They aren't for the U.S. models but are on Canada and Europe versions I believe.

Sometime long ago someone on this forum posted a video of them in action, kinda neat looking but not really necessary

 

Those are escape hatches for the 'rotary' chipmunks that inhabit the engine compartment. RX-8s, in case you didn't know, have two of those exercise wheels attached together to provide the motive power for the car. Every so often the chipmunks need to take a break from their work - the square patches are how they exit the vehicle.