I attempted exactly this to tap my ground for my relay for my heated mirror into the ground trigger for the rear defrost, and I ended up frying that circuit in the Entrance Control Module. Either the circuit is really sensitive to extra loads (like really, the 60mA from a relay coil?) or it may of back-fed power into the ECM looking for another ground and blew it. If you do, I'd look into putting in a diode so that it doesn't happen if that is the case.
This. I haven't dug into the heater circuit, but if you use a relay with a transistor based circuit, you _NEED_ a diode in reverse across the coil so it doesn't spike current back to the transistor. The diode doesn't need to be anything huge, I've typically used a 1n4148.