Just to clear it up completely, I take it that the LEDs are driven back-biased, then?
Forward Bob.
If fitted the other way, no current flows through the LED.
It only takes a low voltage to drive current through a diode, so for use at a nominal 12V they need a load resistor (in series) to drop most of the 12V supply (and reduce the currentso the diode does not burn out).
The parallel bypass resistor is so that sufficient current flows for the flasher relay to see a standard incandescent bulb type load. If not, think about when a bulb fails in your indicator circuit - you get a rapid flash (which many fail to notice!). Same effect if you were to fit LEDs with no ballast (assuming you don't have one of those smart flasher relays that will drive either bulb or LED).
Rob, you'd have to go through the Construction and Use regs - light positioning...
