With the analogue Varistream, to avoid problems, I have always fitted, close to the unit, a metal "toggle switch" to put the vairistream on and off.
The switch also is very visible from outside the van if you are looking to see if you have actually switched the unit on.
Cut the red power wire going in to the Varistream and reconnect each of the 2 cut ends ( one to each of the 2 terminals ) at the back of the switch. Make sure you buy a metal one as the plastic breaks easily and also only one that has 2 terminals on the back,( not 3)