I believe there are 4 bolts on the back that hold the bezel on, you should be able to get in there, just need to crawl under there and reach your hand way up in there.
Maybe I shouldn't say this out loud, but if you carefully pry out the glass of the bezel, the Omix Ada Jeep CJ7-tach fits perfectly! And to 1/10th of the price of an original tach! Now they are very similar looking, maybe about 25% smaller.