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Tha is for that. I inherited an hp tuner setup with a license available, do you think that would do it?"The 1999 trucks (GMC or Chevrolet) dont have the option to self disable or even enter the mode. The option was added in the 2000 model year. To disable (only disable - not set any other way.) you have to go to the dealer and have them reprogram your BCM (Body control module). Its under Gm Bulletin #99-08-64-003A published 9-27-99. Go to the dealer and give them that bulletin #. It depends on how well you are with the dealer if they are going to warranty it. The bulletin reads as it can be done under warranty once on a vehicle. I know i am a new user, but i have been working at a chevrolet dealer since before that body style truck came out and remember that being an issue."
you can read more here, it is an off site click, hence I linked it to the word "here."