If you are familiar with GM vehicles, you may have heard about a system called "Theftlock". It's basically a anti-theft system that prevents a radio from working after it is jacked from the car. Obviously, you are going to need to disable this system before you remove the old head unit. So what you need to do is put the original head unit back into the dash, don't really worry about screwing it in, it just needs to be connected to the harnesses for this. If you used a wiring harness adaptor like you should have, it should be an easy thing to do. =P
Anyways, I'm pretty sure these should work for all Theftlock systems. Since you've already cut the power to the deck without disabling it's anti-theft system, you are going to need to reset it's Individual ID number, which you need to unlock it... You will probably have to look in your owners manual for this or contact the manufacturer. Here, check it out:
Unlocking the stereo after power loss(Do this first!!!!)
1. When the power is restored to the stereo, the stereo won't turn on and LOC will appear on the display. Enter your
ID code as follows; pause no more than 15 seconds between steps.
2. Turn the ignition switch to ON, but leave the stereo OFF.
3. Press the MIN button. '000' should appear on the display.
4. Press the HR button to make the last two numbers appear, then release it.
5. Press the HR button until the first one or two numbers appear.
6. Press the AM/FM button. If SEC appears, the numbers you entered were correct and the stereo will work. If LOC is
displayed, the numbers you entered were incorrect and the anti-theft feature is still enabled.
Theftlock - Disabling the anti-theft feature1. Press the stereo's 1 and 4 buttons at the same time for five seconds with the ignition ON and the stereo power
OFF. The display will show SEC, indicating the unit is in secure mode (anti-theft feature enabled).
2. Press the MIN button. The display will show '000'
3. Press the MIN button to make the last two numbers appear.
4. Press the HR button to display the first one or two numbers of your code. The numbers will be displayed as entered.
5. Press the AM/FM button. If the display shows '__,' you have successfully disabled the anti-theft feature. If SEC is
displayed, the code you entered was incorrect and the anti-theft feature is still enabled.
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I suggest that in the future, you ALWAYS search Google or post here! Especially with GM cars. You normally won't have to worry with pre 1994 non-GM vehicles, but anything from about 82 and up and get very tricky with GM thanks to their anti theft stuff... like the VATS system and Passlock I & II. Not trying to lecture you, but it's honestly just a really freakin' good idea. I do it myself all the time. So anyways, I hope this helps point you in the right direction buddy. Isn't car audio fun?
--mindctrl