I've done some extensive searching to try to find which wire to splice into for the AC clutch positive wire but haven't found an answer. Does anyone know/remember if it's the solid black wire or the black/yellow wire?
Solid black is almost always ground. But get yourself a test light or a voltmeter (you shouldn't be wiring up the new fan without one anyway) -- and probe each wire; at idle with the a/c on/clutch engaged, you'll see which one is ground and which one is hot. Tap the hot one for your fan.