Has anyone used the Nissen heater core in their Mirage?

[Casey]

I've been driving my mirage a few years without heat with a bypassed core. It began leaking a few months after I bought the car used. It had been replaced once already under warranty by the original owner. I got tired of not having heat and bought a Nissen core since it had the different clamping style hoping it would be better. I took it to a dealer for replacement because the job seemed overwhelming for me. They say they can't get it to not leak at the fittings and want to put in an OEM core. Has anyone used the Nissen core and have advice?