Look, if your celica has an overflow tank, then it has a radiator cap; I can't help it if you know nothing about your cooling system, or cooling systems in general.
If you're too lazy to look at the top of your radiator to see the cap, it doesn't make it fictitious. A cap is a required thing, it's not an option, it's what regulates coolant pressure in the cooling system, and it's there to cover the hole for a pressure tester, to check for leaks.
When you drain the coolant to replace it, how do you think you fill the system up again, from the overflow tank? please! it wouldn't happen if the system was bone-dry, you have to fill it from the radiator, then top the system off through the overflow.
The reason they tell you to only add coolant through the overflow is because too many people try to top off through the radiator fill, but they don't wait for the car to cool down first, in other words, too many people either getting blasted with steam, or worse, pegged in the head by a radiator cap and a face-full of boiling anti-freeze. Not everyone should do their own cooling system maintainance I suppose.
Our radiators are mounted low and at an angle, try taking off your plastics, and follow that overflow tube to the filler neck on the radiator, the cap is shaped like charlie brown's head. don't believe me? That's your problem, not mine. I'm just trying to help out would-be do-it yourself types. You can disbelieve in radiator caps if it makes you happy.