will differ depending on your LED specs and battery voltage). I twisted the leads together for better connections, cut them shorter as needed, taped it all up with electrical tape (OK, I've left out some missteps), and popped it in in place of the trunk's tubular incandescent 12V bulb, in such a way that the unconnected LED lead on one side and the unconnected resistor lead, made contact with the metal contacts. Works just fine. Rather dim in daytime, and I haven't tried it at night time. The old bulb drew 0.7A. The new LED system draws much, much less. So no danger of running out the car battery if I leave it on overnight.
I still need to do this to the dome light. My reading of Texas law suggests that alas I am not allowed to do this for the puddle lights ("running board courtesy lights" must be amber or white--maybe this only applies if they're on while the car is going, but why take risks?), so when I go observing, I'll just put some tape over them.
The cost for the whole project was low. I had the 330 ohm resistors lying around for years (I think they're 1/4W; ideally, I should have used 390 ohm). I got the LEDs for about two cents a piece from Tayda Electronics.
There was one unexpected cost. I blew a fuse when removing the original bulb. Lesson: Keep the fuse disconnected when doing this, except when measuring stuff.