Replacing globes with LED's in turn signals requires you to either replace the flasher with one suitable for LED's, or put a "load equaliser" in
parallel with the LED's. This applies even if you replace just the front or rear, tho if you're going to do some you should probably do all, just to gain the benefits of LED's.
You'll also probably find that if you're putting LED's in turn signals you'll need to modify the turn signal warning lamp too by adding some isolation diodes to it, or you'll end up with 4 way flashers instead of turn signals. For more info about this, see
this thread and
this one too.
