my car will suddenly die when driving in hot summer. No warning lights come on.

I think you have a fuel pressure or ignition problem. Get it checked by a Mercury/ford specialist. shouldn't be hard to diagnose. Are you asking about the heater core? It has nothing to do with your problem. If you want to find a specialist, here's a directory link for you: https://repairpal.com/mercury-repair-in-claremore-oklahoma

I agree, a heater core will not cause an intermittant stall condition as you are experiencing. Sounds more like fuel or Ignition, meaning ignition module, cam and crank sensors, ignition coil, fuel pump for example. I would recommend you get a professional diagnosis from a dependable shop, BBB listings are a good way to find reputable shops. Good Luck.