The overwhelming majority of "LED bulbs" on the market are fraudulent, unsafe junk. Here's what to know about LED retrofit bulbs in vehicle signal lamps (brake lights, tail lights, parking lights, turn signals, etc).
First, the quick nutshell version:
-Answer to "Will it work?" is a whole lot more complicated than just yes/no.
-Fundamentally different kind of light source, so unlike with filament bulbs, physical fit doesn't guarantee optical compatibility or acceptable performance.
-Giant mountain of unsafe junk on the market, all fraudulently hyped as an upgrade.
-There are a few legitimate products that work OK in some of the lamps they fit in; important to check the actual function carefully.
Longer version:
Start with how signal lamps (parking lamps, turn signals, side marker lights, brake lights, tail lights, reversing lamps) have to work. It's a lot more complicated than just "Yep, that lights up and looks good to me". Please take a look at this post, which goes into detail on how exterior safety/signal lights have to work. I'm point you at it not because I think you should go to the trouble of the involved measurements described (that was more for his build-from-scratch project), but just so you get some idea of the complexity involved beyond "Yep, it lights up".
Bulb-type lamps (brake light, tail light, parking light, turn signal, sidemarker, DRL, whatever) rely on a point source of light, a glowing filament, that radiates more or less equally in all directions—a sphere of light—collecting and distributing that light with optics in the lens and/or reflector. A group of LEDs can't duplicate this at the required scale, so the light distribution from bulb-type lamps equipped with "LED bulbs" like this is often seriously damaged.
The few legitimate LED bulbs that exist work well in some of the lamps they fit in. In others, they work poorly. Their performance has to be carefully assessed in whatever particular lamp they're installed in, by comparing them side-by-side with the original incandescents as reasonably well described (in an accessible DIY manner) here .
And in still other lamps, they don't work at all, because all the legitimate LED retrofit bulbs have only rear/side-facing emitters. That works in some lamps with a reflector bowl behind the bulb to gather and magnify the light (such as '64 Valiant tail/brake lights), but there's a whole other kind of lamp that doesn't use—or doesn't only use—a reflector bowl, such as the '64 Valiant park/turn lights up front. Instead, these lamps have Fresnel-type optics, the kind where the lens has a central magnifying area directly in line with the filament of the installed bulb, and spreader optics surrounding that magnifier. Often the magnifier is a round bullseye and the spreaders are a series of round prismatic rings surrounding the bullseye, but sometimes the magnifier is square or rectangular and the surrounding prisms are rectilinear. If there's no light directly out the front of the LED bulb, there will be minimal to zero output from the lamp.
If you want to try this, the Sylvania Zevo bulbs already mentioned in this thread are the (only) legitimate ones worth a try. That's these.
You will need a different kind of turn signal flasher to operate the LED bulbs correctly; that's this one (2-prong like original; connect its ground wire conveniently).
Whether you wind up using LED bulbs or sticking with the spec filament items, give the lamps a fighting chance to do their job for you: clean the lenses in hot soapy water. Stuff wads of masking tape in the bulb holes, or remove the sockets if they're the removable type, clean the reflector bowls with alcohol, then paint them with Ceiling White paint (specifically! Not just any old white paint; go to a paint store and get Ceiling White), which is practically ideal for the task and does a better job than the chrome/aluminum types of paints.
