honest answer to your problem, there is no single solution, in due time the crack will just keep on stretching till it hits a curve, there is absolutely no permanent fix to it, best is to just purchase a dash cover to avoid the sore sight of the crack. Other approach like putting on silicone adhesives and the like will work only for a short period of time. (Its physics and chemistry at work) with time lapse, the material use in your dashboard dries and stretches as heat is applied. Sorry about your dash, next time do take good care of your dash by using preventive maintenance solutions.