1 opaque and heat-concentrating container. I suggest a tackle box.
1 large magnifying glass attached to a heat box. A front windshield in a black rental car works great.
1 hot Summer day
Several hard Runts. Starting with the chewy variety is just cheating.
Put Runts in tackle box. The more direct sunlight the runts has, the softer they'll get, so you may want to only have one or two layers of runts in there. Place the tackle box on the front seat in your car. Don't worry if it's in shade initially - if you position your car right when you park, the sunlight will come to the Runts. Opening the windows a crack seems to have no effect, so long as the day is hot enough. Let simmer for about four hours - in my case, that conveniently worked out to be exactly two class periods.
Warning: Do NOT attempt to eat the Runts right away - they are hot, and will likely burn a hole through your soft pallate. At least, it'll feel that way.
Allow to cool. Don't worry about the outer shell becoming discolored and splotchy - that's a sign that the process worked. Once they're back at room temperature, pop one in your mouth and enjoy the soft gooeyness. It's not gummi bear consistency, but it'll certainly make you say "hey, this used to be a hard candy! What the heck?"