Just recently the heater core on the inside of my 65 mustang on the passenger's side has been leaking for no reason. I tried to use a radiator sealant but after a while it started leakin that stuff too. Can somebody tell me what's up?

 

Heator cores over time get corroded and rust out. Thats why it's leaking. The only fix is to put a new one in.

 

^^^ What he said. Also, once you replace the heater core, take it in and have the cooling system flushed. A lot of times those radiator sealant/stop leak products will clog up the system. Typically you want to avoid using stuff like that on a car you want to keep.

 

Ditto what they both said, but make sure you aren't just leaking from the hose connections first.