I'm wiring the camper lights on my Fuso FG. Truck has a service body with single turn/brake light. Truck is Japanese so came with separate turn/brake lights so service body has a trailer adapter in the wiring to make the single turn/brake light work. All lights work fine on truck and service body. On camper tailights work fine. Right turn/brake works fine. Left turn/brake doesn't work. On the left turn/brake I have voltage coming thru the wiring (about 5 volts--same as right side and same as voltage on turn/brake light on the service body). If I put the left turn/brake light bulb in the fixture at the camper, voltage at the plug in for the camper drops to 0 but turn signals continue to work fine on the truck and service body. If I remove the camper left turn/brake bulb I can measure the voltage at the fixture, but as soon as I reinstall the bulb, voltage drops to 0. Why does installing the bulb cause the voltage to disappear? I've tried switching bulbs and it doesn't make a difference.
I'm not that great on diagnosing electrical problems and this one has me stumped.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13