No longer drive a Miata, although I still love them!
Not a mechanic, although I do date one!
He says that a relay is a pretty cheap thing to buy just to check, but that it could be the motor. Something about the polarity and that's why it works one way... I can't even quote all technical stuff he yammered off, but if he gets a chance, I'll ask him to post to you when he gets home.