Vehicle- 1969 Charger
Problem- Short somewhere under dash?
How I got here- Issue began with left rear turn signal and brake lights inoperative. Car had multiple issues but where I'm at now is that when I apply the left signal and check with a test light at the kick panel connector (and I have the connector disconnected, I am testing on the side going to the underpass harness), test light flashes strong on both tight and left signals at the correct terminals. Running lights to rear light test lamp at correct terminal when headlight switch is on.
However, when the left signal is activated, the test light will also flash weakly when the probe is touched against the terminal coming from the fuel gauge.
Using a multimeter, I get a weak but audible tone when I check gor voninuity as gallows. One lead of multi meter to fuel gauge terminal and one lead to left rear turn signal AND with one lead going yo terminal for running lights.
I'm stumped. Is this a bad ground causing a backfeed or a short in the dash somewhere or am I being punished by dome satanic vilt?

what does the fuel gauge have to do with rear lights?

fuel gauge feed pulses because it is 5 volts but from a pulsing voltage reg at the dash, unless you changed it to solid state.

It's called back feed.

As the flasher for the lights pulses it loads and unloads the entire electrical system and you are seeing that back fed into where ever you are testing.

Bet the engine is off while you are doing this.

Yes, to each So basically what you are telling me is that what I am seeing is normal?

Are you using LED brake/tail lights? I had a similar problem when I had 4 LED tail and brake lights (1968 Charger) the lighting for a tachometer I have installed on my column would light up when I had the breaks on. I chalked it up to some kind of backfeed problem Because of problems with the flasher, I've had to switch out one of the LEDs on each side, and I haven't noticed the tach lighting up anymore

Did it do t new, NOT.
It is not normal and you probably are having a grounding issue, clean the metal up where the light socket plugs in and see if that fixes it.
Electricity is like all liquids, they take the easiest path back to the source, the battery

No LED on mine. Incandescent bulbs. Correct type for each socket. Harnesses are not hacked into. No high amp alternator. It's largely as Dodge made it.

That's what puzzles me. Fuel gauge of course has nothing to do with turn dignals. That's why it seems strange to me that a test light applied to that terminal flashes in unison with the turn signal flasher and only with activation of the left signal.

Try this......It's possibly a bad ground connection so run a wire from the metal casing of the brake light / turn signal housing inside the trunk to a good ground such as one of the bolts that fasten the trunk lid latch. This worked for me on a '70 Road runner with similar issues.

it sure sounds like a ground issue to me.
also, look over the front turn signal light/housing assembly for a good ground as well.
you just never know about grounds.

Tail light, brake lights, and turn signals, and front turn signal lights on a 69 Charger are grounded through the taillight (or front turn light) mounting screws. Paint, rust, corrosion, or even dirt between the screws and the sheet metal the screws are in will cause all kinds of strange things to happen. The power for those lights is back tracking through the lighting system looking for a place to ground. That often comes through dash, or instrument light grounds.

The best "fix" is to solder a ground wire on the bulb holder and attach it to its own ground, or you can buy replacement bulb holders with a ground tab attached you connect a ground wire to. You will probably need to do this to each bulb in the tail & front turn lights. The headlights are usually grounded by their own dedicated ground wires per side of the car.

Sounds like it's a non-issue to me. He's only seeing something with a test light.

If that's the case then

The fuel & rear light wires are in same wire bundle going to rear of car.
Something frayed along the way back there maybe?

If it's an LED test light, and I am guessing it is, it's doesn't take much to make them flicker.