Prepare and bake cake in a 9x13" baking pan as directed on back of cake mix.
Let cake cool. Using a fork, poke holes all over the top of the cake. Spread whipped topping over the caramel layer and sprinkle with crushed Heath bars.
Mix together the sweetened condensed milk and caramel topping and pour all over the cake, letting it sink down into the holes.
Store in refrigerator.
Notes
The good thing about poke cakes, is that they taste better with time. This is a cake that could definitely be made the night before serving. I wouldn't recommend making it more than 24 hours in advance, or it may get a little too soggy.