I wish to hardwire the accessory tail light on my e-bike. It uses 2 AAA batteries at present (3V). I consider a flashing tail light to be critical to safety. It has died a couple of times during a ride, unbeknownst to me. My thought is to use a buck converter from the 48v main battery (or the 6v output supplied by the Bafang wiring harness) but I don't know the requirement of the light in amps. Any advice is welcome.
To all practical purposes, they use such a small current it can be considered none. AAA Batteris are around 750mA, lights last around 30 hours - around 25mA. Running off the 6V supply will require nothing more complex than a simple 3V regulator.
It is possible to buy lights specifically for Bafang, which would be a better option if your EBike is compatible with them. If you cannot find a set, IMHO, running two rechargeable battery lights and regularly recharging them would be best.
There are tail-lights that are designed to be powered by the e-bike battery. At least Shimano and Bosch units provide either 6 or 12 volts DC to the taillight through a buck converter. Many taillights designed for e-bikes accept both 6 or 12 volts DC.
Use the 6 volts provided by the Bafang wiring and buy an e-bike specific taillight. It's intended for powering lights. A second buck converter would just add to the idle power consumption of your electrical system. It's better to use a single buck converter for many things than to add yet another buck converter for yet another accessory you want to power.
The amperage is about zero. Long time ago, the German light system had 3 watts at 6 volts (0.5 amperes), out of which 2.4 watts (0.4 amperes) were for front light and 0.6 watts (0.1 amperes) were for taillight. That was at the time of incandescent and halogen bulbs. Today, I'd say 0.01 - 0.03 amperes is a reasonable draw for a taillight. If powered by the ebike battery, that probably has 500 watt-hours, then you probably could get over 2000 hours of usage, but the ebike system probably has higher idle current draw so the high idle current would cut that battery life to a fraction.
In any case, the e-bike propulsion system uses way more power. Good systems cut propulsion when you have only few percent battery left, leaving the rest for the lights.
With e-bikes, you should probably throw away the flashing function. It makes it hard for car drivers to observe the path of the light, because at one instant the light is at some point, and then it's off, and then it goes on at an entirely different point. Humans are better adapted to observing motion of lights that are continuously on rather than flashing.
The only benefit of flashing is if the flasher is powered by few small batteries such as 2 AAA's. In that case, flashing increases battery life. If powered by a dynamo hub or an e-bike main battery, flashing has no usefulness.