I'm wondering if her remains were in the driver's seat or the passenger side, or the trunk. (Which I'm doubting). Had her seat belt been buckled or was it too destroyed to tell?
1). If there was a perp, where did he put her while driving to this location? Or did he make her drive, disable her at the scene, position her in the car and then torch the car?
2. Or did she pull off the highway because the car was making unusual sounds and then parked in a driveway to perhaps call for help? Did the car explode before she could get out or call someone? Was her phone found nearby?
3. Or did she pull off in a secluded driveway and set the car on fire?
A lot will depend on where she was in the car, if she was seat belted in, if her phone was nearby, where the fire started (could she have set it herself) and was an accelerant used, (if so, what kind and where?) If an accelerant was used, and she set it herself, was there a can nearby?
Just thinking out loud about possibilities.