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There are 3 places, the bottom/back of the unit on the sticker, a yellow sheet that came with it new and the last is on the side of the box it came in new. If you have none of the above you will need to contact garmin directly at:
Monday - Thursday: 8 AM - 6 PM, Friday 8 AM - 5 PM Central Time (closed holidays)
US:(NNN) NNN-NNNNbr />US Toll Free: 1-800-800-1020
If you need further assistance just let me know.