Coolant leak - Stop Leak

I have an '89 190e, 2.6 that has developed a small coolant leak. Most mornings I find a small puddle of coolant beneath the car. The leak is far back, and seems to be originating somewhere near or behind the firewall. I suspect the heater core. Has anyone got any advice regarding how to find or test for leakage in the core?

Also has anyone had experience using the stop-leak products on the market? I think I might try disconnecting the heater hoses from the engine; flush the heater core and run some stop-leak through the heater core only to see if it does any good. Any thoughts or comments?

Thanks