Hello, my 1984 dt100 has developed the embarrassing habit of flashing the tail light when I use the indicators. What would be the most likely source of the problem? I charged the battery up today and it’s was still doing it. 

My first thoughts are a bad earth somewhere there with either the tail light bulb socket or the loom earth if flashers have a separate earth wire into the loom. But I guess the flashers just earth onto their mount?. Loom earth usually ends at the coil mount under the tank although if that was bad or loose then you'd be chasing other strange electrical problems. Also check the tail bulb because I had one once that the stop filament had lent over & touched the tail filament causing me great grief at the time.

Good Luck!
My DT2 started doing the same thing and I can't figure it out. Voltage to everything seems fine. My turn signals are NOS and look like new inside and out. The socket inside my taillight looks as good as new with absolutely no corrosion. I've cleaned the ground at the coil, I've replaced the taillight bulb, the battery, the taillight pigtail, the main wire harness, the flasher with a NOS one, both the front and rear brake light switches, added a NOS voltage regulator, and just had DEET restore my left side handlebar switch. My taillight still flashes with the turn signals.
Next will probably have the ignition switch restored...

I agree that it is likely a bad connection, and might even be on one of the lives on the bottom of a bulb.

You could try removing one bulb at a time, to see what effect this has.

It's likely dimming as the signals flash due to voltage drop in the wiring or grounds... or a battery that doesn't have the power to maintain voltage under the load.

It's not that hard to check voltage at the battery terminals with a meter with taillight and signals on to see how much the voltage drops across the terminals when signals cycle "on".  Very often at the battery posts themselves you'll see like a 2 volt drop as signals cycle on because the battery just doesn't have the capacity to maintain voltage under load. 

If there is minimal drop right at the battery, you can also check the voltage at various points in the taillight circuit as signals cycle. 

Grounds are checked with same method.  One meter lead to battery negative post itself and then other lead to ground at the lights.  Meter should stay at zero

I like using an analog meter to see meter needle deflect.

To illustrate how this works, you can unplug the taillight connector.  Connect one meter lead to the female and the other to the male.  Turn on taillight.  With the meter being the only thing connecting the two wires, the meter will deflect full battery voltage (and taillight won't light). The meter reading is the voltage drop across the "connection"...  and since there is NO connection with the wires disconnected, voltage drop is 100% as shown by the meter.  If the wires were connected and you used your meter on each side, you should not see the meter move at all.  But say the meter shows 1 volt.  That means you are losing one volt across that connection. 

Or you can just keep guessing and buying parts. 

Many thanks, will start this on Saturday. My problem is that  I am like many emotionally attached to the bike, so I hate it when it’s poorly! 

Just a random thought but if the battery wasn't a grunty enough little one those have, like CCA cold crank amps in a car battery & you run with the lights on, does it try to top up the draw from the ac headlight that's running. Or is your tail light ac too like ours here in oceania. In other words does your headlight dim along with tail when flashers are operated. Are you running a high wattage relay & bulbs. Are your headlight & tail higher wattage than original. I guess your headlight is sealed not bulb replaceable like we have luckily.

This happened to me on my 76 DT400 (or maybe it was my DT2), I figured out that it was a bad front brake switch. Hope that helps!

Hello, if this adds any info the neutral indicator on the speedo also flashes as does the high beam bulb indicator. Taking out the rear bulb makes no difference to the front bulb flashing alternately to the indicators???……

Bad ground between handlebars and main frame?