That board is using the diodes as "regulators" but depends on the incandescent large bulbs as load, to then push the diodes towards the maximum 0.7V silicon diode drop https://en.wikipedia.org/wiki/Diode.
Again, here is the concept. You have the load of the regular large incandescent bulbs in the car for main lighting. These diodes are in series to those lamps. Across the diode, as you load it, the voltage increases to a maximum 0.7V per diode.
Forward threshold voltage for various semiconductors
- Silicon diodes: 0.6 V to 0.7 V
I count 6 diodes. Let's say they wired them in pairs so one always conducts the AC waveform. That logically puts 3 diodes in series.
3*0.6= 1.8V and expected- many grain of wheat bulbs are only 1.5V
So again, you only see 1.5V when the circuit is loaded by the larger incandescent bulbs. If one or two burns out or are not connected- then you get near nothing out the terminals for the grain of wheat bulbs.
