This is about replacing a LUCAS FL5 3-prong flasher unit with a plug-and-play replacement circuit that will work with a much lower current than the FL5.
The FL5 uses a bi-metallic strip and a heater coil to provide the flashing mechanism, see Diagram.
When the load is applied (there is in fact the indicator turn switch between the load and the FL5) the heater warms. This causes the bi-metallic strip to bend and the contacts close. This shorts out the heater, and applies full current to both the load and the test light. As the strip cools the contacts open again. This repeats, and the lights flash.
The unit is NOT polarity sensitive and was used in both Positive and Negative Earth Cars.
Using LED replacement bulbs reduces the current through the heater to a level where the unit will not function.
So we need to replace the heater with an electronic equivalent.
Issues here include no permanent connection to both +VE and -VE (depending on the car polarity) and the need to make the circuit polarity insensitive.
You should note that there are commercially available units BUT these either are only for -VE cars or they have another flying lead that needs to be earthed. Being a purist I really want the replacement unit to be exactly the same as the original, only electronic and not electro-mechanical.