02 ss 3.8l NOT supercharged
Car sat for probably a year with no battery. Installed a known good battery. Have 12.6v at battery. No light in interior. Only check engine light comes on. No dash lights. No "clicking" when ignition on. I checked all fuses. All important fuses have 12.5v. Verified good ground cable continuity. I don't see any mouse damage or anything.

Any ideas to try?

 

Just to be 100% sure I'm following - you've confirmed that all fuse boxes are receiving 12 volts?

I know you mentioned checking the important fuses, but wasn't sure if that was just in one box or across all of them.

 

Check for signs of rodents and chewed up wires.