Mazda Miata: Radio Anti-Theft?

Since I don't have the owner's manual and everything I have read says NOT to
disconnect the battery unless you have the code for the radio system. Now
our 93 Red was bought just two months ago and had been through at least 5
owners since it was originally built and naturally, there is no owners
manual with it.
I want to clean the battery terminals...is there a generic code that will
get the radio to work afterward?
Would I have to take it to Mazda to have them reset it?
TIA
Tom

Thanks, Jim, this information was very useful and enlightening. Just what I
needed.
Tom