If you know a few things about your system, i.e. where the GPS and accelerometer are on the rover/with respect to one another and which way the accelerometer is facing on the vehicle, this becomes much more manageable. If you know which way you're travelling and which axis experiences the acceleration you can start to deduce heading. Careful placement of the accelerometer and a little calibration re. how turns show up in the data should get you started.
We'll likely offer an IMU module at some point which will help with this considerably.