What's this heater core you speak of? Is that the big black thingie under the dash that blocks access to wiring and hasn't been connected in years?

Since I did the work in the rain last night, I stuffed a garden hose in each side of the block and let it flow until the water came out clean. There was no coolant in there at the time so it was no biggie. I flushed it heads first then through the block.
I'll pull the drain plugs and do it again to be sure everything's out. Getting rained on is one thing but I didn't care to lay down in it and get that kind of bath.