quarter panel or what?
 

I have an SA and heres the problem
http://i42.tinypic.com/2dmh9mq.jpg

theres rust behind the wheel on both sides and the bottom of the rear is just about all rusted out what should i do?

Depends on how much time, money, and skill (or skilled friends) you have. The correct way is to cut all the rust out, pretreat the metal with phosphoric acid and weld in new sheet metal (lots of threads on this) but it takes time and money if you don't have acces to a welder. If you're not planning to use the car as a dd and no structural components are compromised I would look at what Ray Green did (he epoxied in the new sheet metal and shaped it with fibreglass-reinforced bondo), which is what I did a year and a half ago and its holding up very well (though to be fair the car is only driven in the summer and garage stored in the fall). You have to be very meticulous about removing all the rust (I made sure I had 2 inches of clean metal before I did anything) and treat anything that you can't remove with phosphoric acid (including the bare metal as it shouldn't be exposed to air for too long.

Someone, just recently, posted a link in this or the tech section of a guy from the UK that's making the lower rear quarters and fender lip. That would be you're best bet for the fender skin. For the inner wheel wells, no one makes those. You can make or have them made. Fiberglass has been done, but without perfect prep work, life is limited for that repair.

Here's one link I found. http://www.alfaparts.net/mazda.html Not sure if this is the same one or not.

what about at the bottom of the rear bumper like beside the bumper shocks?
the reinforcement is all rusted through =/