Just curious, and I'm thinking the answer is yes (cause the factory stereo is), but is it OK to attach the metal case of an aftermarket stereo to the stock metal bracket to the frame (ground) of the car. This won't cause any problems will it?

Right on.

One question, the green wire in the factory harness is, as I understand it, the antenna relay. Why does it get 12 V when the ignition is on? I was under the impression that the head unit simply uses this wire to send it's 12 V signal to the antenna to tell it to go up....is this a function of the whole Mazda negative signal operates the antenna situation? For whatever reason my blue acc wire isn't getting 12 V either, so I was thinking of using the green.

Right on again.

 

bizzump