Battery's
 

I know this is probably a stupid question, but I have not owned a 250 d 4x4 before, nor had batteries in series. So my question is, if I have to replace one battary do I have to replace both?

You may not necessarily have to replace both batteries but you should have the other battery tested to be sure it still holds a charge. However, if both batteries are old and you have not ever changed either then there is a good chance that both batteries should be changed because in a series hookup, if one battery gets drawn down to where it no longer holds a charge then the other battery(s) end up making up for the loss of voltage from the obviously bad battery, and then they are at least somewhat damaged and will prematurely draw down any new battery put into the system.

Thank You