If your heater core blows up, and develops a leak, here is a simple way to bypass it, until you can replace it. Which not is a simple job by any means. Of course, for those of us here in Texas, we only need the core for like 2 days anyways! :-D
Facing the car, with the hood up. You can find the heater core (located under the dash) on the passenger side, near the top of the firewall. They are quite a bit hard to reach. I had to pull my distributor cap, in order to reach one of the tubes. You might not have to. The connection tubes exit the firewall about an inch adapt. These will be the two hoses you need to connect to each other.
You'll need to get a heater hose connector, 5/8" to 3/4". I got mine at Autozone. There is no brand name on the yellow pack. Just p/n 5103. In any case, you'll need a connector that adapts the two different size hoses together.
It's really that simple. Connect the two hose ends to each other, to bypass the core. They are not easy to reach though. There's a bunch of wires running right in front of them.
In this pic, you'll see one hose identified by the red dot. Follow this hose to the firewall. The other hose is slightly lower, and to the right. That hose make an immediate turn down, about 2" from the firewall.
All this was done on my '87 GTA. On my past '90 RS and '92 RS, they were the same. I've heard, early years may be different hose routing, and not have the vacuum assist stock bypass, for when the heater is turned off. But, the heater core will be in the same location, so it is essentially the same procedure. Even with the heater tuned off, the water can still reach the heater core. If there is a leak inside, like mine, water will run out onto the floor inside your car.
ATGO is not responsible for any damage done to your vehicle by following the above instructions.