There's two lines that run into the firewall that are the send and return coolant lines to the heater core. Drain the coolant, disconnect the lines ( on the passenger side of the firewall, right behind the throttle body) and by some little plastic heater hose connectors from any auto parts store. Just connect the two lines. Mine is bypassed, and I used a heater hose connector that is an elbow to do it. Took 10 minutes lol.
If yor heater core is bad one of a couple things will happen, either when u turn the defroster on it will leave a film on your windshield from the coolant fumes, or on the passenger side of the firewall towards the bottom you will see what looks like a fitting for a hose to connect to. It is a weep hole and coolant will leak from it. These things only happen if it ruptured tho. If its clogged they won't.
1992 Fox 5.0 S/C 347, The Beast!
1995 GT 5.0 all stock, 168k miles, the beater!
1998 Cobra, Atlantic Blue, 21k orig miles, all original!