[phillyg]

Disconnect the red and black wires from the negative post. Install a knife switch on the post, and reinstall the red and black wires on the other knife switch terminal. Don't know who installed your cables, but it's wrong to have a red wire to a negative post, and note the red wire to positive post is retaped as black. Makes no sense, but as long as they're working, I'd just do the knife switch.