HI,
I too wanted to replace the crappy radio in my Fuzion 371 so I did just that this week. Looks from pictures of your Carbon 31 that the same could be done.
I bought this unit on Amazon. I wanted the Bluetooth, Better sound, XM, etc.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Then I cut out the opening to fit the Double Din sleeve. Then hooked up the wires, however the Radio Trim Benzel needed to be filed down to fit right over the radio and up to the wood.
I also replaced the outdoor speakers with these Kenwoods:
https://www.amazon.com/Kicker-41KM65...keywords=KM654
These sounds Amazing.
Now next step to the problem. I am going to build a box to inclose the speakers from the outside which is behind my couch. Then I will add a switch and two speakers in the coach for indoor listening without the outdoor speakers on.
Not sure why Keystone cant do this at the factory !!!
Best of Luck
Before and After