Charging System Troubleshooting--Blown Ignition Light Fuse?

[ACHiPo]

When I was driving yesterday I noticed the ammeter was at -25A. When I got to where I was going I unplugged the fan and the ammeter jumped to +25A with the engine running without the fan. I plugged the fan back in and drove home. The ammeter read 0 when the fan wasn't on. When the fan turned on the ammeter pegged negative before coming back to -25A.

I spoke to Peter and Doug this morning. Doug pointed me to the troubleshooting instructions on the ERA website. One of the first steps is to confirm that the ignition light is on when in the Accessory position. It wasn't. I checked the wiring diagram and found the fuse. It is blown. I replaced it, tried again, and the light came on briefly then went out. The replacement fuse was blown. The fan circuit fuse is fine.

It seems it doesn't make any sense to further troubleshoot the charging system until I find and fix the short that is causing the ignition light fuse to blow.

Suggestions on where to look?

Thanks.