Your local boating supply store may have an alternative for
     the interconnection. For about $35 you can get a marine
     dual battery switch that would look quite professional installed
     somewhere in the seat base.

     It has four positions: "Both" (for those cold weather dual battery
     starts), "1" (for using/charging the first battery), "2" (I'll stop
     with the obvious), and "Off" which disconnects both batteries from the
     system, perfect for outsmarting the typical car thief. There is also
     some sort of protection so you can switch from one battery to the
     other while charging without blowing out your alternator.

     Tom F.
     90 Syncro Westie (Labrador Bound?)
     90 Audi 200 TQW IA Stage I