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If YOUR radio "came right up" after disconnecting the battery, then YOUR radio did NOT have the security system on in the first place.
Don't give people the false impression that C5 factory radios will all work after a battery disconnect.
Evil-Twin is CORRECT in his statement.
It's not that they did not have a security system in the first place, it means that it was never activated. And it is officially called "THEFTLOCK." So it means that the previous owner(s) never activated the Theft-Deterrent Feature. Refer to Chapter 3 of your owner's manual.
I wish it were true that the square area you describe would actually direct the acid away from the PCM (main computer), but it does NOT. All C5's are made that way. The square is the battery support post for the battery tray, but it does nothing to redirect leaking battery acid.
Many Corvettes have had extensive damage to the computer, wiring harness, vacuum lines, and etc. from battery acid.
Before purchasing a C5 it is important that the area under the battery be thoroughly inspected first.