Pin 3 to ground is the way to do it. Not sure what pin 2 is. It could be ground.
If you don't read 70% (or 30% which is the same thing with a different point of reference) when checking (key on engine off), try reading voltage. A ratio of measured voltage to battery voltage times 100 will give duty cycle. In other words if you measure 6.5v and battery is 13 the ratio 6.5/13 equals .5 times 100 equals 50%.
Bosch Master, ASE Master, L1
33 years MB technician