ok so i have a problem like this on my s4 na gxl '87.
i had one light out but both work when the brights are on, i pulled the 15a fuse, and now, its reversed, the other one only works but when the brights are one they both work. i believe there is a short. please and thank you i don't want a ticket.

 

The Red/Black wire to each headlight is the wire which delivers voltage to the low beam headlight filament. So, you need to check to see if both of these wires has voltage when they should. This R/B wire comes out of the Dimmer Relay before splitting into two wires with each wire running to one of the headlights. Seems like your problem is related to where the wire splits in two. If this is not the case and your problem relates to a grounding issue of the R/B wire then an ohm meter will help you identify where the shorting of the wire is occuring. So, check for voltage on both wires and go from there.

 

it turns out the bulb was bad. thanks