When installing a battery disconnect switch on a car battery you should install the battery disconnect on the positive terminal of the car battery.
However it really doesn't matter which terminal you install the battery disconnect switch on because it will stop the flow of electricity either way and if you have the negative disconnected the battery will not supply power to the vehicle anyway.
Because with a battery it requires both the negative and positive battery cables be hooked up to allow the flow of juice through it.
I've always just installed the battery disconnect switch on the positive which is what is recommended but you can install the battery disconnect switch on any terminal that you want to and it will work just as good.