91 GT-S help

I'm trying (keyword trying) to install a radio in a 91 celica gt-S. The problem is I couldn't find a wiring harness anywhere (gt-s is too rare). I figured hey not too tough I'll just splice it in. I did this on my Honda and many other cars with no problem. So I have all the wires out and big problem. Two hours with a voltmeter and I still don't know where the 12v+ is. All I'm sure of is the ground (big gray wire).

So has anyone done this? Does anyone know what color wires are what? Or do I need to run to the dealer crying.

Thanks

Figured it out... whoever had this car last had the factory CD player swapped out at the dealer for a casette. GR. The OEM wiring harness was taped WAY (and I mean WAY) back in the dash. Found that, and it made my life a whole lot easier. Went to the dealer and got the wiring diagram life was much better. BIG mess. ugh.