Trial and error. The bulb can go in if you turn them 180 deg. I put LED in the holder with same polarity on same side.
If you're smart enough, you can find out resistor in the photo. Also LEDs should be current limited, not voltage. As long as voltage is about 0.7V it'll turn on. Amount of current control brightness. Also don't try to lower running current too much thinking it'll help with longer life, it doesn't. Under current LEDs will over heat.
Damn, who's the EE here?