Well - I can deduce...
Knife was likely inside the car and not taped to the outside, so the knife was in the car. Passenger kicked the door - something that would be unusual to do from inside. Witnesses saw this happen, which makes it more likely to be outside.
Conclusion from a Thai Visa certified detective: Passenger kicked the car while cabby was inside. Cabby they got his knife, got out of the car and stabbed the passenger.
You can take that to the bank ????