There should be enough oxygen in the exhaust to at least ignite the propane and support a modest burn. It won't be very hot or reliable, but as soon as it leaves the tailpipe it will give you the desired result.
If you're having trouble with not enough oxygen, drill a hole in the exhaust upstream of the propane, then point the drill bit (still in the hole) toward the back of the car. This will make a little reverse scoop that will pull a little air in as the exhaust rushes by.
And (although I'm sure you know this) your little addition could land you up to 30 days in jail since its a felony in almost every state. In some states even more since not only are you in possession of an illegal incindiary device, you constructed it yourself. Its like the difference between underaged possession of alcohol vs. underaged moonshining
