What causes the electric door locks on a 2001 Grand Prix chatter while driving?

Found it! After changing the Park/Neutral Position switch (the only common switch between the locks and the truck release) I checked the voltage at the trunk release relay. Only got 3.3 volts instead of 12 volts. Traced the problem to a corroded wire in the bundle which runs above the battery. Spliced in a new segment and fixed all the problems. Kudos to ALLDATAdiy.com for having the schematics and component locations I needed to fix this.