Serious electrical / audio problem?

O.k., I bought a '95 3.8l convertible from my friend. He mangled the wiring under the dash and doesn't have the factory amp or radio anymore ( or harnesses ) so I took a sony head unit that was sitting in my garage and tried to install it. I've installed radios before ( not Sony ) and did a pretty good job but this has me stumped. I know most people use a bypass harness but since it plugs into the amp ( which I don't have ) I figured I have to do some rewiring. I found the speaker wires and connected them just fine. When It was time to do the power, i had no problem finding the constant and accesory wires. Here's the problem....The harness that the stock radio plugged into has THREE grounds, and there's ANOTHER 12v constant coming from under the driver side dash ( I assume to power the amp ). I capped the extra constant wire and connected a ground to the sony harness and it worked! Problem is, my airbag light started flashing and the next day, my car wouldn't start unless I turned the headlights on first! After that I tried connecting and reconnecting grounds and I think I blew several fuses. Does anyone know about the wiring on this car? I can't find any schematics detailed enough to trace the problem. I think that the 12v constant that went to the amp somehow completed a circuit through the radio (for the airbag?!) and now it's open. Also, what do I do with three grounds when there's only one wire for ground on the sony harness and no illumination wire ( there is one on the car though). I'm sorry this post is so long but I wanted to describe the problem in detail. Thanks in advance.
p.s. now my car won't start. Accessory works but not start.

Here's a little extra info...I hooked a voltmeter to my lighter's ground wire and checked for continuity in all three ground wires from the car's radio harness and they're all good. There's also continuity between the mystery wire that went to the amp and the other 12v constant that goes to the radio. From what I've been told, the amp had two harnesses that both went to the radio. One harness was just the 8 speaker wires that hooked into the back of the radio. The other harness had an amp remote wire ( blue ), a constant 12v ( yellow ), and a green and black ground. The 12v went under the driver side dash, the amp remote to the radio of course, and the ground to the harness above the radio that the radio hooks into. The power harness on the back of the radio had a 12v constant ( green ), a 12v acc ( yellow/black ), illumination wire ( blue/red ), of course the blue amp remote wire, a black ground and another green/black ground like the one that went to the amp. I guess my question is what do I do with the yellow 12v that goes under the driver side dash and the extra grounds? What about the illumination wire? The harness on the sony radio only has one ground wire of course and no illumination wire. I know this post is crazy long but please bear with me here, I'm trying to give all the information I can. Thank you.

Thanks for replying, I'll try that. What about the constant 12v that went to the amp? If all it did was power the amp, do you think it would hurt to just tape it up and out of the way?

o.k., I'll try to just cap off the extra 12v and the other grounds. I'll let you know what happens.

No, it's nothing like that. I think I know what happened with the airbag though. Someone told me that when you disconnect the battery on these cars the airbag diagnostic module stores a code in memory and keeps flashing it until you reset it. I just have to find out how to reset it, so I don't think it had anything to do with the radio. Here's the wiring scheme: ( the in car harness, I don't have an amp )

Constant---Green
Acc-----------Yellow/black
Ground------One black and two Green/Black that connect to each other ( one went to the radio along with the black ground and the other green/black went to the stock amp )
Illumination-Blue/Red


I don't know off hand the color of all of the speaker wires but I traced them to each speaker with a battery and marked them. The harness for the head unit has a constant, acc, ground, amp remote, ant. remote, but no illumination. That's all I know. I'm sorry, I'm an idiot when it comes to anything electrical. I can pull an engine and rebuild it but I don't fool with anything electric. Electricity hurts.