Detonator attached to small container of gasoline, hidden either under the seat or under the dashboard. Either remotely command-detonated, or timer-detonated. More likely command-detonated from someone in a following car
She drives along, then BOOM, fire in the car, she loses control, and burns
Who needs C-4 or Semtex, you can just buy a gallon of gasoline for a buck and do just as well.