You back out of the driveway off Woodward on a hot July morning and there it is — a small orange-pink puddle where the front of your MINI was parked. That's coolant. On a MINI Cooper, a leak like that most often traces to a cracked plastic thermostat housing, and Pontiac's summer heat is what finally splits it. It starts small. A slow drip, a faint sweet smell, maybe the temperature gauge wandering higher than usual. Left alone through a Michigan summer, that trickle becomes a roadside overheat and a much bigger repair. If your MINI is leaking or running warm,