929 Madza (1992) No Heat

I have a 929 Madza 1992 year. Does not blow any heat. I noted that the 30 amp fusible link for the heater was busted. Replaced it but still no heat. A bust fuse means an electrical overload somewhere, what do I need to look at to ID my problem?

I have change a little heat filament up in side the blower and I have changed the heater control unit on the dashboard.

My suspect the heater core but I need some help before I full this care apart.

Thanks.