If it is a 230 volt circuit wired with 12/2, you cannot use one leg plus ground for a 115 volt application. You should run a separate circuit for the lights as tapping a 115 volt application off of a 230 volt dedicated circuit is also not correct, even with a neutral present, unless the 12/3 ends in a sub panel from which the heater and lights are fed.

Thanks randy, I'll grab some power somewhere else because I'm not sure if I have room for another breaker.