2002 GMC Sonoma Owner's Manual - Page 214

THEFTLOCKR is designed to discourage theft of your radio. The feature works automatically by learning a portion of the Vehicle Identification Number (VIN). If the radio is moved to a different vehicle, it will not operate and LOCKED or LOC will be displayed. When the radio and vehicle are turned off, the blinking red light indicates that THEFTLOCK is armed. With THEFTLOCK activated, your radio will not operate if stolen. Theft-Deterrent Feature RDS Radios (If Equipped) Understanding Radio Reception AM The range for most AM stations is greater than for FM, especially at night. The longer range, however, can cause stations to interfere with each other. AM can pick up noise from things like storms and power lines. Try reducing the treble to reduce this noise if you ever get it. FM Stereo FM stereo will give you the best sound, but FM signals will reach only about 10 to 40 miles (16 to 65 km). Tall buildings or hills can interfere with FM signals, causing the sound to come and go. 3-60