TimC writes:

On 2013-08-13, James (aka Bruce)
was almost, but not quite, entirely unlike tea:
Ah, well that's good. Perhaps the light housing has a voltage
limiting zener diode or similar, to protect the 3.6V bulb from the
full 6V. Or perhaps the original lighting circuit had the front and
rear bulbs wired in series, so that the 6V was shared across two
bulbs. I can't tell from here.

LEDs of different types *have* to be wired in series (the current
source will just double its voltage output in order to maintain the
same current going to 2 equal LEDs in series). You can get away with
very similar lights being in parallel, until they age. Then one will
start taking more current, until the power supply starts overpowering
it, and there'll be thermal runaway and that LED will completely short
out, rendering you with a Dark Emitting Diode.

FWIW, when the front halogen bulb was absent, the rear LED still worked
fine -- and it would even activate a bit (maybe blinking, I don't
remember, it's normally constant-on) when backing up, which it doesn't
do with the working bulb in the front.

Based on my high school electronics, I took that to mean they're wired
in parallel.