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The security light issue is very common on GM vehicles. This is from a break in the communication between the key, igniton, and TDM (Theft Deterrent Module) computer. The hard part is figuring out which one is failing. They is no way to know other than to just start switching out old parts and putting in new ones. This is expensive and all parts must match for the security to work. There are ways to bypass the system Other than that, you'll need to take it to a dealer to get FIXED. If you want to bypass the system, you can go to this link for all the info you will need. Ive bypassed dozens of these things and its a pretty easy DO IT YOURSELF fix. http://vatspasslockpasskeysecurityhelp.yolasite.com/
With one speed working it is more than likely the blower motor resistor (20 dollar part) located in the area of the blower--needs air to pass by it as it gets rather hot), or at worst case the Module. This part (Blower control Module) is a bit more expensive. Try the resistor first, you can test its resistance at different speed settings to see if it is functioning as it should.
This is definitely the Relay which is part of the Blower Motor Resistor Module. The Engineers designed it to FAIL when the Return Spring breaks. The Relay Contact is only utilized for the Highest Speed, The armature (Movable Contact) makes connection due to gravity when the Spring Fails. The Relay is mounted UPSIDE DOWN and was designed into the module that way. It is not independently serviceable!
Take it from me as my 2004 Silverado has gone through 3 Resistor Modules and 1 Blower Motor. Michigan winters are relatively cold!
Your problem is the Relay which is encased inside the Heater/AC Resistor Module. This is a very common problem with GMC and Chevy Trucks. The Relay's return spring fails and the relay is held closed by gravity. The only fix is to replace the Resistor Module. I have a 2004 Silverado and have replaced the Module 3 times and the Blower Motor twice. The Orange wire feeding the Module is always Hot (12V) and feeds the Relay Contact for Highest Blower Speed. Hence the Blower runs all the time even with the Key off and removed.