In glass bowl, microwave raisins in rum for 30 seconds. Cover, let sit until rum is absorbed, about 20 minutes. In small bowl, sprinkle yeast and sugar over 3 tbsp warm water; let sit until yeast begins to foam, about 10 minutes. In separate bowl, whisk together flours, ginger and salt. Scrape yeast mixture into separate large bowl; mix in eggs. Beat in molasses. Gradually beat in flour mixture until dough is elastic. Beat in butter until thoroughly mixed. Mix in raisins. Scrape batter into well-buttered 10" tube pan, smoothing top. Cover with kitchen towel and let rest in warm place until doubled in volume, about 2 hours. Bake at 375°F until skewer inserted in cake comes out clean, about 22 minutes. Run knife between cake and pan; turn cake out onto rack, let cool. Rum Syrup: Place sugar in saucepan with 3/4 cup water; bring to boil and stir in rum. Place cake on plate, top down; spoon half of rum syrup evenly over cake bottom. Flip cake over. With a sharp-tined fork, poke holes all over top crust. Spoon remaining rum syrup over top of cake until all the syrup is absorbed. Let stand 2-3 hours before serving. Store at room temperature for up to 1 day.