Just added Hella 500s to my pickup. The install went pretty straightforward, but when flipping the switch only one lamp lit up.

Is one lamp working an indication of correct wiring/circuit-function? Maybe just be a bad bulb?

 

Both lamps have to be grounded check that, if you didn't know. It could also be a bad ground on one of the lamps. You might also want to check the connections on the relay. I went ahead and purchased a new relay with my kit because the hella directions are to confusing with me and i didn't want to mess something up.

If you think it is a bad bulb you can swap the bulbs around to check that, or use a voltmeter