Heater Core leak
If the heater core is leaking you will find coolant on the floor board of the passenger side. The Core is not located in the area of the drain tube, the drain tube is for water that can come in through the cowl from where the fan get its air. The heater core is just out side this area. To replace it, sorry, you have to pull the dash to remove the heater then remove the core from the heater. You can try pepperica about two table spoons into your radiator and and bring the water temp up to normal, this should stop up the hole at the leak if it is not to large, pin hole size no problem, 1/8" won't work. I have used this in the past with a lot of success as I have driven a FORD for many many years and not all where First On Race Day. Also Opels since '72.