need to CHECK the CHARGING CIRCUIT  charge the battery if possible(may be scrap due sulphated  plates) start the engine ,with the engine running put a digital mutli meter(DMM) set on 20v DC  Across the battery terminals should  see 14.5v on a 12volt system 8.5V ON A 6VOLT  System with ALL electrical things turned ON anything less than the supply voltage  be it a 12v or a 6v system  then you have an ALTERNATOR problem can be with  the regulator faulty, or may have slip ring brushes that are worn but this is not a major problem.
                 wirecutterman
                
 September 2008