Dave asked, -at the very top- why the radio locked.

Mine locked when the car battery suddenly died. I'd bought the car used, and the previous owners had emigrated and had changed the code. And you get four chances before recoding becomes a very expensive operation.
I found that it is possible to re-code the radio yourself, sort of, but that involves removing the radio and sending it to an independant specialist, refitting it and using the new code. And that costs more moola than a new radio does!
Does anyone want to buy an old Toyota radio and CD-player set with one chance left?
The new radio is the type that takes one CD at a time, in a slot on the dash, and NOT lying in the boot/trunk loading six at a time.
The whole idea is an anti-theft device, radios being in the past a prime target.
Also, changing the battery engages the burglar alarm, which can only be switched off with the remote, and you cannot change the small battery in Toyota remotes of this vintage, which involves getting a re-key specialist out to your car.....
Make sure your remote works.