Preheat your oven to 325 degrees F. Spray a bundt pan (with at least a 12-cup capacity) with cooking spray (or grease and flour pan).
Evenly sprinkle pecans in bottom of pan.
In a large mixing bowl, mix together the cake mix, pudding mix, eggs, water, vegetable oil and rum with a mixer on medium speed until smooth.
Pour batter over pecans.
Bake for 50 to 60 minutes, or until toothpick inserted is removed clean. Place on a baking rack to cool.
Prepare the glaze while cake is cooling. Melt butter in a small saucepan over medium heat. Add sugar and water and stir to combine. Increase heat to high and bring to a boil. Boil for 4 to 5 minutes until thickened, stirring frequently. Remove from heat and slowly add rum (exercise caution when adding rum, mixture will bubble). Stir to combine.
Once cake has cooled, invert onto a serving plate. Using a skewer or toothpick, pierce holes throughout the cake. Drizzle glaze over tops and sides of cake, allowing it to absorb the glaze.
Serve and garnish with maraschino cherries and/or grapes, if desired.