Well, you already know C-8 is correct so you're doing something wrong. Nothing I have shows A-10 being used.
Verify on the cluster which is the switched 12 volt. Either eyeball it or ring it out using the meter. Since it feeds most of the gages it'll be obvious. Iirc it's even marked on the board as "IGN" but don't hold me to that. Once you ID it trace back to whatever connector and pin it goes to (should be C-8) and take it from there. Once the other side (coming from the fuse) is verified wire it to C-8. I think you made a mistake though.
Again, any time something doesn't make sense or a signal is missing immediately stop and do whatever it takes to verify you're probing the right place.