Also if I unplug the interface from the car, car will start..

 

​​​​​I got mine used and it also had a relay. I believe the new ones also come with a relay. I left mine where it was. Sounds like you have a short somewhere/ something isn't wired right/ bad head unit or interface. Mine was pretty much plug and play except for the volume control I missed 😂. I can pop mine open tomorrow and get some pics for you so you can compare. Is the head unit new? Have you tried getting the instructions for the interface online and going over them?
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The relay is what interfaces the BCM to the radio ON switch.

You might note that there is no "switched" input in the harness.

GoOgle will get you an installation manual. I know because I did that to research this thread last\ year.

For those too lazy to type "gmlan 04 manual" into a GoOgle search: https://www.google.com/search?q=gmla...hrome&ie=UTF-8

 

I got the manual from the Crutchfield site. When I follow it step for step, car still just stays in security mode, nothing else turns on.

 

Possibly the used interface is toast?

 

Thats what Im starting to wonder as well. Wish I knew how to test it before grabbing a brand new one.

 

There's probably a way to test it with a multimeter. Did you check to see if the relay was bad?

 

I​ just had another thought. Disconnect the plugs from the interface and make sure there's no corrosion anywhere. Also, check for any corrosion on any connections that you did not make yourself. Assuming all of yours are good.

 

That will be the next thing I'm doing, checking for corrosion, bad wires etc. Then hook it up and checking to see any power even coming through it.

 

So wife and I brought in the Pioneer CD player and GMOS Interface. Made sure each wire was color corrected. Wire nut and Tape each wire to make sure secure connection. Follow the step by step manual to get it all installed, still jumps into security mode.

If I plug into the car, but not the harness, car will start, once you plug in the GMOS interface, car goes into security mode... No voltage on the radio side. Everything seems to go wrong at the interface...