From what your saying and what I can see from your pictures they maybe held in like rubber mounted windscreens where the rubber has a groove that sits on the metal cutout, to remove would require you to push in one corner of the rubber from the inside to unhook it from metal and slowly work along top, once top and side is unhooked you can normally lift out , two people one in one out is best in case unit drops out and rubber seal may have mastic or silicon around it to form water tight seal, beware metal edge maybe sharp and possibly may need to replace rubber seal depending on age/condition