I have found the easiest and cheapest way to disable the LOCK on the 2005 model radio from a Chevy Avalanche.
Most of the posts are showing to remove the security chip, thus destroying a lot of functionality.
The best way is to cut the power on pin 8 of the security chip. No need to unsolder the pin and possibly destroy the chip. Use an Exacto knife to cut the circuit board trace to just that pin, or use a dremmel tool with the cut-off wheel to cut just pin 8.
I got this radio from my son's 2006 Avalanche after he replaced the radio with an after market. I installed it in my 2003 Silverado 2500 (had a bad after market in it from the previous owner). Everything works fine, except for the calibration error message on power up that goes away.
Bill L.