And the firing from impact is not heat- it is percussion. All explosive materials can be detonated by shock- how MUCH shock is determined by the sensitivity of the explosive. Things like Compostion C-4 are very insensitive- shoot bullets into with no effect. Others, like Fulminate of Silver are SO sensitive it cannot be handled at all without going bang. Primers use lead azide, which is pretty touchy- a good impact is the energy donor to cause it to fall to its lower energy state- releasing energy as it does so.
But unless your car is on fire, it should not make a primer fire. Think about all of the ammo in the sun, in an ammo can, in a truck- in Iraq.