How do you know what type of hose or clamp to buy when a 1996 Corisca overheatsand all the antifreeze leaks out when the heater is on?

If your Corsica is leaking Radiator coolant when you turn on your heater it is probably a rotted heater core, but may just be the lines. If your Corsica is overheating even when you put rad fluid in it, then it may be a more serious problem, such as a dead rad fan, or mal-functioning thermostat. You can ask at any local shop, or even Canadian Tire as to what size hoses and clamps you may need, and any other replacement costs or other information you may need. If you are not sure if it is the heater core behind the passenger side of the dashboard, then you may want to have an experienced friend or well trusted mechanic check that for you.