Chances are that if its a dash piece its either ABS or PVC - not polyethylene. Either ABS or PVC can have parts attached using crazy glue and baking soda. Glue the pieces with crazy glue (you don't have to wait for them to set). Put a small bit of baking soda around the break then add crazy glue so that it soaks into the powder. The powder should turn from white to almost clear if the glue soaks in properly. Keep this up till you're happy with the buildup. It can be drilled, tapped, sanded, painted etc. and will yeild a virtually indestuctable joint. The baking soda actually reacts with the crazy glue so you may see a bit of smoke.

You can get ABS or PVC cement in the plumbing section of any hardware store but the above method will outperform either.

If something doesn't "harden" its not a glue, silcone is a sealant - not a glue