Help! Front marker lights out, but turn signals are working?
 

Need some help troubleshooting the front lights on an '87 US Carrera... Please bear with me :)

The side marker lights are out as well as the right front, but the left front works. Both front turn signals and headlights work and the rear markers/TS/brake lights also work as they should. Fuses 5, 6, 9 and 10 are good and I've cleaned the connectors in the front housings and replaced all of the bulbs.

I'm not an electrical wiz, but I did take some measurements. I'm just not sure how to interpret them.

Right TS to battery ground = 12.8v
Right marker to battery ground = 12.8v
Right TS to ground connection at the headlight = .04v
Right marker to ground connection at the headlight = 0v

Left TS to battery ground = 12.8v
Left marker battery ground = 0v
Left TS to ground connection at the headlight = .01v
Left TS to ground connection at the headlight = 0v

I've been racking my brain on this for several hours and I'm stumped. If it's a bad ground, my TS and headlights wouldn't work, Right?

I thought this was unique to 86 only but this may be your problem. There is a diode that controls only one side marker light. In my car it is under the dash a touch left of center of the steering column.

The part # is OEM-911-617-108-00. The thing has an opaque plastic case with a wire going into each end. You open the case and you will see the diode.

I just replaced this in mine and it worked for the first time in 2 years.