Replacing Head Unit

I don't believe it. I found the problem.

Today I took the car to the shop to deal with a transmission problem. While I was there, I brought up this problem. Naturally, I plug the new HU in, and it works!! I go back home to finally finish this nightmare, and it doesn't work.

Here is where the problem was: The new HU came with a sleeve. You mount the sleeve to the car, then slide the HU in. However, in my car, there's nothing back there on which to mount the sleeve. So, I took the 90° angle brackets off the old HU, and mounted them on the new one, not using the sleeve. Apparently, there's something involving the sleeve and the clips mounted to the HU that causes it not to work if not connected.

Which leaves me back at square one...trying to figure out how to mount it WITH the sleeve intact.

Thanks to everyone for their help in this so far!

Mike