Radio Wiring Harness - Big Mistake

I've make a stupid major error and am hoping someone out there can tell the best course of action. I was trying to install a new cd player/radio in a 1999 Buick Century Custom. I bought the radio, dashboard adaptor, and wiring harness adapter at Best Buy. After taking the old radio out and soldering the leads from the wiring harness adapter I purchased to the harness from the new radio I went to connect it to the factory wiring harness. They didn't match, they weren't even close.
They label on the package said it was for a 1988 to 2002 Buick Century, but it was totally different than the old one.
Here is where I made my big mistake. I thought OK, I'll remove the factory harness head from the wires in the car, and simply connect the same color wires from the radio harness to the corresponding color wires in the car. I made this decision without checking that the wires in the car were the same color as those on the new radio's harness. BIG MISTAKE!!
Not only weren't the wires exactly the same colors, there is no red wire (usually the power), there is no plain black wire (usually the ground), and there are two IDENTICAL yellow wires!
Can I get a wiring diagram on line somewhere that will tell me what the codes are for the factory wires that used to go to the factory radio harness? How can I tell the difference between the two yellow wires? Should I go to the dealer and buy a new radio wiring harness to reconnect to the wires in the car, then go back to Best Buy and demand a wiring harness adaptor that will go from the new radio to the car wiring harness?
Any help anyone can give to an old do-it-yourselfer would be greatly appreciated. I've put many 8 track, cassett, and cd players in cars and trucks in the past 30 years and have never run into this before. I assumed the wiring harness adapter would have the correct head to match the factory wiring harness, and I further assumed that the wires would be the same color. Bad assumption!

Had a similiar problem 2 weeks ago, I installed a 93 Lumina delco radio in my 88 Century. Didn't match up, I had to use a reverse harness. Anyways I got it done, used this website to get the factory wiring diagrams for GM radios WWW.mnrelectronics.com and went to the link for installation help. This site should help. jag21

Had a similiar problem 2 weeks ago, I installed a 93 Lumina delco radio in my 88 Century. Didn't match up, I had to use a reverse harness. Anyways I got it done, used this website to get the factory wiring diagrams for GM radios WWW.mnrelectronics.com and went to the link for installation help. This site should help. jag21
Thanks for the url, I'll check it out. I ended up going to Circuit City and getting the correct wiring harness from them. I didn't actually use it, because I had removed the factory harness. However, it contained a schematic of the purpose of each pin in the harness and I used that to connect to the wires still in the dash. I could determine the color of the wire that had connected to the corresponding pin of the factory harness by the remnants of the wires still attached to the head. Only the two yellow wires were a question because I had a 50-50 chance of quessing right and connecting to the correct one. I removed the fuse box from inside the dash so I could access the back side of it. I was then able to tap into the accessory power wire as it left the fuse box. There were at least 4 wires in the factory harness that had no corresponding wire in the Circuit City adapter. I just left them unattached. Everything works fine so I guess they aren't critical.
A word to the wise: NEVER buy an after market radio or car CD player from Best Buy. They don't know what they are doing. G0 to Circuit City. They do.