Lets clear up the misconceptions about that fuel gauge. This gauge has 3 contact studs on its backside where a typical gauge has only 2. Reduced voltage from a gauge voltage limiter goes in through the typical gauge and out opposite stud to a sender. Those 2 post gauges are mounted to the housing in such a way that they are isolated from chassis ground. So the reduced voltage can't go anywhere other than where it's supposed to go. The 3 post gauge has 12 volts going into it via the 3rd post at switch on because the voltage reducer or limiter is mounted inside that gauge. So this particular gauge must be mounted different from those others because reducer/limiter needs a chassis ground, or it doesn't function properly either. If you were to remove the fuel gauge from the housing yyou would find a slither of metal on its backside and a small unpainted spot in floor of housing for that slither of metal to contact. That is the 1st step of more or less same daisy chain of metal-to-metal contacts that create a ground path back to the battery negative post. Now I'm wondering do any of your gauges work? I'm also wondering why you don't have/use a proper meter. The basic test lamp might show you enough ground to light its tiny bulb yet not nearly enough to make a entire inst' panel work.
Anyway... back to my point here, I sincerely doubt the fuel gauge can have any effect on how turn indicators, illumination bulbs, etc..., work or not. Hope this helps.
