I replaced the 3rd brake light on my 2004 Greyhawk also. It was an early LED light that was sealed. I found a suitable replacement at etrailer.com. Measure the size of the light and distance between the screws holding it onto the back of the MH. You will need to use the proper type of Dicor caulk used for vertical surfaces (not self leveling).
2016 Greyhawk 29MV
Wherever you go, there you are!