The article linked below contains generally good advice for international travel and is worthy of your read.
The only thing I don’t always agree with is the author’s advice to sit in a cab directly behind the driver.
It depends on the situation. In some neighborhoods, cab travel is too expensive for the locals. If they see an obvious passenger, they know that guy has money. In those neighborhoods, sitting up front with the driver makes the locals think he is driving a friend and not a customer with a bunch of cash. That may save you from a robbery attempt.
When sitting in the back, I like sitting opposite the driver. Any carjacking attempts are going to be coming from the driver’s side. Being able to exit with the cover of the car between you and the carjacker is a good thing.
Besides that, your most likely threat in most third world countries is the driver himself. Sitting opposite the driver makes it a lot harder for him to impede your exit should you have to bolt.
Want more info on taxi safety? Read my tactical training scenario about the topic.
Around the Globe: Tips to Make Your International Travel Safer