Q: Could it be a fault behind the dash somewhere? SOmeone mentioned a common earth? Help!!!
A: The common ground is a possibility. Follow the negative lead on your battery down to where it grounds to the vehicle frame (bolted solid to a big chunk of steel). If that connection looks like it has burned spots such as black sooty looking places, or is corroded with salt grime, dirt, etc. it needs to be unbolted, all surfaces cleaned, and reconnected. Check also your battery connections to make sure they are clean and tight. Acid corrosion on the leads will lead to the same thing. Good luck!