I already had this,
Power Probe running off my adjustable power supply to test 6v & 12v bulbs, flashers, Led's.
and have also used & tested mech units in the machine or bench testing mechs & electric motors/gearbox's after rebuilds
you can trigger pulse the supply for accurate testing.
this is something similar on ebay.
ebay.com link: Automotive Power Probe Digital Multi Tester 6v 12v 24V 4 Professional Mechanics