Add the sultanas, 1/3 c apple cider, rum, and vanilla to a small bowl or jar. Cover the container and allow to sit overnight.
Preheat oven to 350F. Grease a 10-12c Bundt pan.
Cream the butter and sugars for 3-4 minutes using a stand or hand-held mixer, until light and fluffy. Add eggs, one at a time, mixing after each addition. Whisk together flour, baking powder, cinnamon, nutmeg, and salt. Add dry ingredients to the wet, mixing until just combined.
Peel and core the apple, then shred with a box grater or food processor (careful not to turn into mush). Peel and grate the parsnip. Fold the grated apple and parsnip to the batter.
Drain sultanas, reserving the liquid. Add the sultanas and 2 tbsp of the raisin liquid to the batter and mix until just combined.
Spoon the batter into the prepared Bundt pan. Bake for 45 minutes, or until top is golden brown and a toothpick comes out clean. Allow cake to cool in the pan for at least 1 hour.
While the cake is cooling, make the glaze. In a small bowl, whisk the confectioners sugar with the remaining 3T of apple cider (or raisin liquid).
Invert the cake onto a serving plate and pour the glaze over the top.