C5 Back brakes not working

We Have a C5. It was recalled for replacement wheels- apparently the wrong ones were fitted. The car is under three years and has been in and out for repairs over 11 times. Taking the wheels off showed that the back brakes have not been working. The discs are corroded and rusty. It seems we have been running on front brakes. It is under garauntee but not for the brakes. Yet it isn't wear and tear, because the pads are not touching the discs. Can you give me a possible reason for this please. We are being blamed, yet we would not have known if the wheels had not been taken off. Something seems to have stopped the action. I assume the car is still under warranty. It is true that brake pads are wear items, not covered by warranty. The rotors are not covered either for wear, but all brake parts are covered for manufacturing defect.

Brakes that don't work are defective. This is warranty work, and you must insist!