Right!
Thanks. Tried it last night. It seems to be working quite well with one 220 ohm resistor in series with the taillight.
The tip about alternating the direction of the LEDs is a good one, and I wish I'd thought of it before gluing the whole thing together.
I used this formula (and then did some filddling)
R = (Vs - Vf) / If
Where:
Vs = your source Voltage (usually 6 Volts for a bike dynamo).
Vf = the typical LED forward Voltage, find that in the LED's data sheet.
If = the typical LED forward Current, find that in the LED's data sheet.