Another method is to separate the wire that feeds the lamp from the wire that drives the armature and feed it from a separate wire to the flasher units under the dashboard. A flasher unit is required for each side and needs to be able to operate at the small current of the 3W bulb. Note the filament bulb can be replaced by an LED bulb (Part number 7) The old units that normally use the current from 2x 21W bulbs will not work unless you also have flashing filament bulbs front and back as well. If you only want flashing trafficators then a unit suitable for LEDs is required. Part number 1 or 2
The indicator switch is fed by a wire from ignition circuit and is three position -off, left and right. The left and right outputs normally feed each trafficator armature and lamp. They will continue to feed the armatures only but an additional wire from each needs to feed a flasher relay which then feeds each side's flashing lamps (whether just the trafficator lamp or all three lamps on each side).
If you are also going to fit (or already have) flashing front and rear indicators then you need to decide whether these will be filament or LED(Part number 2) . Note that some flasher units are current driven and will not work on both LED and filament but some claim to do both. If you retain the 3W trafficator filament bulb but have LED front and rear then the LED flasher unit should be OK.
Bulbs
Flasher Units
Please pay attention to the polarity of your chassis when ordering units and bulbs. Jowetts may be converted to negative earth.
Note to be within the law in some countries you need to have either an audible or visual indication that the flashers are on. Some flasher units have an audible warning. For visual indication you could wire in an LED or filament bulb as an indicator on the dashboard either before or after the flasher unit depending on whether you wanted the light to flash. You might want to combine these in one location. If you wanted just one bulb then you would need to feed the indicator bulb via a diode from each flasher input or output or use Part number 4
Back in the early 1960's Ben Shaw, who was the first owner of Amy's Javelin, constructed a box of electrics which not only created flashing front sidelights and flashing rear brake lights but also operated the trafficators. The electrickery is still operating today. This is similar to Part number 6