Im not sure what problem you have with your heater core, but in my XJ, i had a very small leak. I could smell coolant in the interior. It must of been minute, but it was annoying.
I didn't want to go down the huge route of taking my dash apart, so i did some research on sealant products.
I settled on this one. Its not like those big black granules you used to get. This stuff mixes in your glycol and you would not know its there. Fantastic stuff and fixed my small leak and never had the problem come back.
https://www.kseal.com/au
You can get it from Supercrap and its worth a try.
Cheers,