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.
Mix together the cake mix, pudding mix, eggs, water, vegetable oil, and rum with a mixer on medium speed until smooth in a large mixing bowl.
Pour batter over pecans.
Bake for 50 to 60 minutes, or until a toothpick inserted is removed clean. Place on a cooling rack to cool.
Prepare the glaze while the 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 very slowly pour in rum (*exercise caution when pouring in rum; mixture will vigorously 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.