Last time, then I'll shut up...
Bare - I know, you didn't promise anything! But you did say that you simply replaced the turn signal indicator bulb with a particular LED and it worked. To me, that's "plug and play", in this particular instance, anyway.
Arizoni, your description is correct - the circuit is designed to work that way... as long as the indicator light allows current to pass "both ways", like the OEM incandescent bulbs do.
b.sheets - the confusion comes because your run-of-the-mill LED does not allow current to flow "both ways", and it won't work as an indicator without the diode scheme, regardless of whether your actual turn signals are LED or incandescent.
The key point, which Gremlin made, is that a special LED unit designed to light regardless of which way the current flows, like, oh I don't know, an LED rated to work with AC or DC, will work in our circuit, which is designed to have current flow both ways through the indicator light. I didn't notice until late in the discussion that the LED's in question happen to be rated for AC or DC, and will therefore work without further modification.
Thank you all - now back to our regularly scheduled mayhem...