Without the switch is dangerous for street use, espically out here in az, it burns up the pumps super quick (within a year with NORMAL driving)
""If you are running a pair of big returnless fuel pumps (i.e. Aviator, Focus, GT or GT500)in your Mustang you will NEED to have your FPDM modified. This modification allows the FPDM to sink substantially more current (up to 23Amps) without overheating. What happens when the stock driver overheats? It goes into thermal limiting and cuts ALL fuel supply to your engine until the driver temperature drops below a safe level. As I'm sure you're aware, the loss of fuel pressure with your engine at full load can often times be catastrophic!""
Strait from maker of the modified FPDM's.