Rear brakes won't disengage when I put it in Reverse on 2006 Cadillac DTS

Parking brake releases when I put it in reverse. Additionally have done it manually to make sure they are released but when I go into reverse the rear wheel brake on the passenger side is still on and tire will not rotate but will only slide. What do I need?

Asked by for the 2006 Cadillac DTS
what do i need?