I use a Auto-Vox TW1. It does however mean that you need another monitor in the car (but it's only a small one). IMHO. it's a great bit of kit - completely wireless at the camera end and excellent battery life. My only real problem with it is that it's a bit fugly. It was an easy option for me as I already had the thing lying around having upgraded my head unit to one with a wired camera facility. Search Amazon for details.