Sorry but I think you'll need an auto electrician for this one.
But first of all, just check that the battery terminals are clean, and there's no movement between the battery connection and the battery post terminal!
It's amazing how many people find that a loose battery connection is the cause of their troubles! If things don't improve, then you'll need an auto-electrician.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix switches and sockets.