what could cause this?

dont forget about the bulk head connection at the firewall they all had issues with that .....the plug end corrode turn green an make a poor connection ....inside the connector its self ..... a little dactylic grease an a through cleaning .. had them even melt the ends out of the block ... one burnt the harness from the headlights the the turn signal switch in the column .... back when that happen you could go the the bone yard an get you one ...not the case now