Make sure you go for superbrites if you are replacing the lamps.
"Ordinary" red LEDs are in the 10-30 mCd range. You should be shopping
in the 500+ mCd range. If you get the 10 Cd ones you may end up with
something that will blind other motorists at night, but you can always
drop the current a little. Each LED will draw about 30-70 mA at around
2V.

You will probably end up wiring them in series-parallel, and you will
find in that case that some LEDs are a little more "greedy" than
others, and will draw all of the current off, so if you plan to use a
series-parallel arrangement, buy extras and then test all of them to
match voltages.

If you don't want to be bothered with all of that, you can just use
more resistors.