Id like to install a battery disconnect that is easily accessible  99 3500 4x4 cummins auto. Any suggestions appreciated.  

Why don’t you figure out where the drain is coming from? 

Inside the cab fuse box has plastic holder ls to pull out draw items like IOD fuse (Ignition Off Draw). Just pull the plastic holder and it will hold the fuse out of the socket for you.

 

@Threadzy is right you should measure all your amperage draw total and on each fuse circuit to find the draw. Typically should be less than 100mA DC or less than 0.1 Amp DC