I removed the panel closest to the windshield and then melted a small piece of the same plastic across the hidden part of the crack. Trim off excess plastic from a non visible location to do this. I used a soldering iron to melt the underlying dash plastic a little an applied the cut piece. Then I heated the piece I applied and smoothed it all along the concealed part of the crack, It is not perfect but much less visible with the entire dash looking again like one unbroken continuous piece again. It as held for more than two years.
If you are not too picky and want to strengthen it more you could run the hot iron over the exposed seam as well but it will leave a visible indent. I chose to make small railroad track like cross laps to strenthen the crack repair.