Thats a function of the BCM. For the 99 year model, you need to take it in to reprogram the BCM with a Tech2, or install a 2000+ BCM.

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You can disable the Automatic (Rolling) Door Lock feature by either recalibrating the Body Control Module (BCM) or with the self-- programmable function. Use the following procedures:

MY 1999 (VIN X)

Important

The feature may be turned off and covered under warranty on a one-time only basis. The system can be re-enabled (turned on) again at customer expense.
1. Connect the Tech 2 to the vehicle.
2. Select Service Programming from the Tech 2 main menu screen.
3. Identify the vehicle (year, type, fuel system).
4. Select Request Information (left soft key).
5. Select Reprogram Existing Module. Verify the Vehicle Identification Number (VIN).
6. Disconnect the Tech 2 from the vehicle.
7. Connect the Tech 2 to the Techline Terminal. Follow the terminal instructions.
8. Select Service Programming System from the main menu.
9. Select Terminal to Tech 2 as the programming method.
10. Select Program Truck Body Controller.
11. Select Calibration with Automatic Door Lock Functionality Disabled.
12. When the action is complete, disconnect the Tech 2 from the terminal.
13. Connect the Tech 2 to the vehicle again.
14. Select Service Programming from the Tech 2. Complete the vehicle ID.
15. Select the Program soft key (second from the left). When the Programming Successful screen appears, press the Exit key.
16. Disconnect the Tech 2.
17. Road test the vehicle in order to confirm that the recalibration was successful.


MY 2000 (VIN Y)

Important

Reprogramming this function for customer preference is not warranty. Please instruct the customers to refer to their Owner's Manual operator instructions or refer to below instructions.
1. Beginning with the ignition in OFF position, pull back on the turn signal/multifunction lever all the way toward you and hold it while you perform the next step.
2. Turn your key to ON and OFF twice. Then with the key in OFF position, release the turn signal/multifunction lever. Once you do this, you will hear the lock switch lock and unlock.
3. You are now ready to program the automatic door locks. Once you release the turn signal/multifunction lever you will have ten seconds to begin programming. If you exceed the ten second limit the locks will automatically lock and unlock to indicate that you have left the program mode. If this occurs, to continue programming your locks go back to step 1 above.

The following is a list of the available programming options:

* All doors lock at 24 km/h (15 mph), only driver's door unlocks when key is removed: Press the LOCK side of the power lock switch on the door panel ONCE, and then the UNLOCK side ONCE.
* All doors lock at 24 km/h (15 mph), all doors unlock when key is removed: Press the LOCK side of the power lock switch on the door panel ONCE, and then the UNLOCK side TWICE. (This is the factory setting)
* All doors lock at 24 km/h (15 mph), none of the doors unlock when key is removed: Press the LOCK side of the power lock switch on the door panel ONCE, and then the UNLOCK side THREE TIMES.
* No doors lock at 24 km/h (15 mph), none of the doors unlock when key is removed (Turns off automatic lock feature): Press the LOCK side of the power lock switch on the door panel TWICE.

You can exit the program mode any time by turning the ignition to ON. The locks will automatically lock and unlock to indicate you are leaving the program mode. If the lock/unlock switches are not pressed while in the programming mode, the respective auto lock/unlock setting will not be modified.[B]