We bought a used 99 Vert.
1) How do I tell if my radio has had a theft code set?
2) How do I get the theft code if one has been set?
3) How do I set or Change the theft code?

 

All described in the manual. Someone else posted a link to an online manual earlier today.

 

Heres the extract:

Theft-Deterrent Feature
THEFTLOCK is designed to discourage theft of your
radio. It works by using a secret code to disable all radio
functions whenever battery power is removed.
The THEFTLOCK feature for the radio may be used or
ignored. If ignored, the system plays normally and the
radio is not protected by the feature. If THEFTLOCK is
activated, your radio will not operate if stolen.
When THEFTLOCK is activated, the radio will display
LOC to indicate a locked condition anytime battery
power has been interrupted. If your battery loses power
for any reason, you must unlock the radio with the secret
code before it will operate.
Activating the Theft-Deterrent Feature
The instructions which follow explain how to enter
your secret code to activate the THEFTLOCK system.
It is recommended that you read through all nine steps
before starting the procedure.
If you allow more than 15 seconds to elapse between
any steps, the radio automatically reverts to time and
you must start the procedure over at Step 4.

Write down any three or four-digit number from
000 to 1999 and keep it in a safe place separate
from the vehicle.
2. Turn the ignition to ACC or ON.
3. Turn the radio off.
4. Press the 1 and 4 pushbuttons at the same time.
Hold them down until --- shows on the display.
Next you will use the secret code number which
you have written down.
5. Press MN and 000 will appear on the display.
6. Press MN again to make the last two digits agree
with your code.
7. Press HR to make the first one or two digits agree
with your code.
8. Press AM FM after you have confirmed that the
code matches the secret code you have written down.
The display will show REP to let you know that you
need to repeat Steps 5 through 7 to confirm your
secret code.
9. Press AM FM and this time the display will show
SEC to let you know that your radio is secure.
Unlocking the Theft-Deterrent Feature
After a Power Loss
Enter your secret code as follows; pause no more than
15 seconds between steps:
1. Turn the ignition on. LOC will appear on the display.
2. Press MN and 000 will appear on the display.
3. Press MN again to make the last two digits agree
with your code.
4. Press HR to make the first one or two digits agree
with your code.
5. Press AM FM after you have confirmed that the
code matches the secret code you have written down.
The display will show SEC, indicating the radio is
now operable and secure.
If you enter the wrong code eight times, INOP will
appear on the display. You will have to wait an hour with
the ignition on before you can try again. When you try
again, you will only have three chances to enter the
correct code before INOP appears.
If you lose or forget your code, contact your dealer.

Disabling the Theft-Deterrent Feature
Enter your secret code as follows; pause no more than
15 seconds between steps:
1. Turn the ignition to ACC or ON.
2. Turn the radio off.
3. Press the 1 and 4 pushbuttons at the same time.
Hold them down until SEC shows on the display.
4. Press MN and 000 will appear on the display.
5. Press MN again to make the last two digits agree
with your code.
6. Press HR to make the first one or two digits agree
with your code.
7. Press AM FM after you have confirmed that the
code matches the secret code you have written down.
The display will show ---, indicating that the radio
is no longer secured.
If the code entered is incorrect, SEC will appear on the
display. The radio will remain secured until the correct
code is entered.
When battery power is removed and later applied to a
secured radio, the radio won’t turn on and LOC will
appear on the display.
To unlock a secured radio, see “Unlocking the
Theft-Deterrent Feature After a Power Loss” earlier
in this section.

 

If it is enabled, (red light flashing), use this link to get the code and reset it.