I haven't done it on a Sundial fixture, but have on others. Its fairly simple.
1. Unplug the fixture
2. Measure the fan and make sure you can get one of same size
3. Remove bulbs to prevent accidents
4. Unscrew and remove fan
5. Cut wires leaving enough left over to work with (extra never hurts)
6. Cut new fans wires (to remove the connector)
7. Strip both fixture wires and fan wires
8. Splice them together and tape with electrical tape ensuring that its not exposed to air (preventing corrosion)
9. Test the fan (if it spins in the wrong direction, just switch the wires)
10. Put it back together! And Voila!