On the back side scrape most of the bondo off the plastic bezel piece and the dash piece. Then get black Abs cement from Lowes. (Comes in a can with blue label in the plumbing area).
Now lay the piece face down and using a small Popsicle type stick apply the abs cement over the area where the two plastics meet. Apply a very liberal amount over the entire area, nice and thick. Let it dry for several days.
What it will do is actually melt and bond the two plastics together making for a nice welded like surface.
After it is nice and dry I would remove the bondo from the front and use bumper epoxy instead. It is more flexible and less prone to cracking. Sand and paint it just like bondo.
This is what I use.
Hardset Filler for the major gap areas. Flex Filler for the final coat to fill the small depressions. Then if necessary you can use spot putty bondo to finish it out.