backwoods fatboy help

I purchased a used backwoods fatboy which I don't believe was properly maintained/cleaned. It has done an excellent job smoking brisket and chicken. However, when I went to go clean it I noticed the holes in the back were filled with soot which looks to have dropped down from the stack. Any suggestions on how to properly clean one of these without losing my seasoning in the process would be greatly appreciated. Photo attached --

Is it causing any problems? i dont know that i could easily get a vacuum in there to remove the soot since it seems to have the packed consistency of a rock. I believe these holes are where smoke exits the cooking chamber and out of the stack. At the present time its probably okay since only the two bottom holes are clogged up. I had thought about popping the screws off and lifting the top/sides off of it to try and clean everything up, but wasnt sure if this would even be accessible.

Great, thanks for the help. I will give that a try and see if it does the trick!