There are no ignition switched 12v fuses under the hood that I have ever found. Use an "add a fuse" and find an appropriate fuse to tap in the fuse block under the drivers side dash. Then run the wire through one of the firewall grommets to the alternator.
@olympic I notice from your sig you have a Mechman alternator as well. Does your harness have this blue wire coming from it as well? After reading all the install threads and YouTube videos, nowhere have I seen anyone mention this wire. Wondering if it's even necessary to hook up as I'm not even sure what it's for.
I bought my alternator a few years back and it did not come with the extra wire. Right after installing it I had problems with the battery light flickering at high RPM and WOT charging issues. Mechman sent me a new adapter harness with the extra wire which I connected to ignition switched +12v and it has worked perfectly ever since.
Did you buy your mechman from me?
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Aside from the fuse block I don't think you're going to have much luck.