Preheat the oven to 350°F (175°C) and spray a 10-cup Bundt pan with nonstick spray.
Make the caramel sauce: place the unwrapped caramels and milk in a microwave-safe bowl and heat on HIGH for 1½ minutes, stirring every 30 seconds, until the caramels are melted and the sauce is smooth.
Cool the caramel sauce in the refrigerator while you prepare and bake the cake so it is cool when used.
In a large bowl, combine the German chocolate cake mix, fudge brownie mix, eggs, water, and canola oil. Use a hand mixer on medium and beat for 2 minutes until combined.
Pour 1 cup of batter into the prepared Bundt pan and set aside.
Stir 1¼ cups of the chopped pecans into the remaining batter, then pour that batter into the Bundt pan, filling it about 3/4 full.
Bake for 50–55 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. If you have leftover batter, bake it in a smaller pan.
Transfer the cake to a cooling rack and cool in the pan for 5 minutes, then invert to remove the cake and allow it to cool an additional 15 minutes.
Make the chocolate ganache while the cake cools: heat the heavy cream in a small saucepan over medium-low just until it begins to boil around the edges. Remove from heat, add the chocolate morsels, let sit 5 minutes, then whisk until smooth and glossy. Let the ganache cool 15 minutes.
When the cake has cooled 15 minutes, poke holes in the top with a skewer and pour half of the chilled caramel sauce over the cake to soak in.
Pour the cooled chocolate ganache over the cake, then sprinkle with the chopped turtle candies and the remaining 3/4 cup chopped pecans.
Drizzle the remaining caramel sauce over the candies and pecans, then let the cake set before slicing and serving.