The jeepney is a public utility vehicle. It does not belong to you even if you are a driver or operator. It belongs to the public—to everyone who rides it and pays for this service. Please do not smoke inside said vehicle, and this goes whether you are a passenger or a driver.
It doesn't matter if you're sitting in the front or at the back—the poison you exhale is very much breathable from wherever inside the vehicle. It's already bad that the vehicular exhaust already pollutes and poisons the air. How much more when you add your deadly vice?
If you want to smoke, do it in a private place or a public place that allows it. Even if there's no sign that smoking is not allowed, it usually means that it's not. Do not implicate others to shorten their lives as well. It's etiquette, and consequence, that when you take a public vehicle, you must respect others who may not share your vices. There might be people who have respiratory complications that take the jeep too, you know?
Then again, perhaps I shouldn't be ranting against you. After all, each stick you consume may be your last.
tl;dr: Don't smoke in public vehicles.