How long have you had this truck? Judging from your description of your symptoms, you have an electrical "mess".
I would disconnect the battery immediately! You then need to obtain a 1948-1951 Chevrolet Truck Shop Manual and study the Electrical Section. Once you get a decent handle on how these babies are wired, you will be able to answer most of your own questions. Then if you run into some weird issues, you can come back and see if someone in here can help you. There are just too many circuits involved to walk you through all of them from afar.
Carl
