In which of the following scenarios would Mechanical Breakdown Insurance NOT apply?

Mechanical Breakdown Insurance is designed to cover the repair costs associated with the failure of mechanical parts in a vehicle, typically due to defects or wear and tear. In the scenario where a rock strikes and shatters Horace's windshield, the damage is a result of an external force and does not pertain to a mechanical failure of the vehicle itself. This is why the coverage would not apply here, as it specifically does not cover damages resulting from accidents or other non-mechanical factors, like impacts from objects.

The other scenarios involve situations where mechanical components of the vehicle are experiencing failure or issues that would typically fall under the coverage of Mechanical Breakdown Insurance. For instance, engine overheating can lead to mechanical damage, worn-out transmissions are a common mechanical issue, and failure of air conditioning systems due to wear or defects also falls under repairs that would be covered by this insurance type.