Does anyone have any troubleshooting suggestions? I'm thinking maybe one of the display cables is bad. It's an old machine and I've already spent almost $100 on replacement parts to troubleshoot.
Step away from the parts cannon.
May I be so bold in saying that you're hoping one of the display cables is faulty because of your sunk costs this point. However, the cost to diagnose/repair will still be that cost regardless of how you get there.
Signs that your Mac is working:
- Boot Chime
- SSH into the machine
These two conditions alone say that your Mac is functioning as it should. The next part of diagnostics is to look at the potential issue and the likelihood it's going to fail. Unless your Mac is slated for extreme environment duty, cables don't fail, thus leaving you with a single component - this display board.
The way you can validate your display board is working is to try yours into a machine known to be good. If the problem manifests there, then you've "brought" the problem with you confirming it's that unit.