Mechanical Considerations:

LEDs have several advantages over traditional incandescent bulbs.  They use much less electricity and can last much longer than an incandescent.  An LED may be able to last for the life of your car.  They are essentially unaffected by cold and vibration.  They can be sensitive to overheating, particularly the high powered ones rated at one watt or more.

Vendors sell LED replacement bulbs that you can plug in just like the factory stock incans, so making a swap can be pretty simple.  Leds are polarized.  They need to have the positive go to one side and the negative to the other.  They won't work backwards.  If you put one in and it doesn't work, try turning it around and plugging it in the other way.

Legal considerations:

It is when you start to deal with the law that you may have problems with LEDs.  Auto makers put a great deal of effort into the design of bulbs and fixtures.  In the US they are tightly regulated by the Department of Transportation, an agency that has long and lovingly detailed lists of rules and tests that every auto light must go through.  Just the rules for acceptable ashtray lighting systems~ 401 National Traffic and Motor Vehicle Safety Act of 1966, Public Law 89-563, 80 Stat. 730, ammended feb 13, 1997, V.7.331 a. ~  take up 1,475 single-spaced pages all by themselves.

When you buy or make an LED retrofit bulb the odds are quite good that the listing will mention in the fine print "not D.O.T. approved" or "Not for on-road use".  This means that *you* need to look over the new bulb very carefully after you get it running and make  sure it is fully as bright and visible from all angles as the bulb it is replacing.  If it is not, don't use it.

It also means that if your vehicle is involved in any kind of incident you have a potential weakness.  When you drive on a public road you are supposed to have D.O.T. approved lighting, and even if yours is fully  as good, that lack of D.O.T. approval could be used against you.

LED "Headlights" and "Fog lights"

You may see listings for these things.  They don't actually exist.  You should read the ad as meaning *decorative* head or fog lights.  If you put them on a bike or electric wheelchair, some kind of slow moving vehicle,  you may be able to actually use them.  On a car they will help let other people know you are coming, but nothing more.  You can't actually drive by them.

Colors:

LEDs come in very pure, very distinct colors that can be hard to duplicate with an incandescent bulb and a filter.  Some of them are quite beautiful.  You might think that any white bulb is pretty much the same as any other, or that all yellows are basically alike, but with LEDs this is not the case.  If precise colors are important for your project you may want to communicate carefully with the vendor to try and pin down the exact color of their product.

In the US and Canada there are a few basic rules for how you can light a vehicle on a public road.  On private property you can light up your car like an exploding christmas tree if you want, and that's your business.  The minute you go out on a public road you should follow these guidelines:

Front:

On the front of the car your lights should be white, yellow, or amber, they must be be placed in matched pairs on the left and right, and they should not be mounted higher than the hood.

Red or blue lights on the front are reserved for emergency vehicles.  Lights that flash, blink, or strobe are also verboten for civilians, except for turn signals.  If you put red, blue, or flashing white lights on the front of your car, you should expect every officer that goes by to write you up for them.

Sides:

Lights on the sides should be amber or yellow.  You may be able to use other colors if they do not upset passing officers.

Rear:

Lights on the back should be red or orangey-red.

Inside:

Inside the car you have more liberty.  You want black lights, traveling red, pink and purple dots, that's your business.  The law seems to permit it.  Flashing/red/blue/white lights should not be visible from in front of your car.

Underneath:

Undercar lighting is new enough that it is not tightly regulated.  Officers tend to like it because it makes it easier to see cars at night and does not blind anybody.   If you want to drive around with a glowing purple or green neon cloud under you, you will get attention, but it should not really upset the law.  Be cautious of red or blue.  Don't have these colors shining out under the front of the car.  If you put in undercar lighting try to put it where you can hose or wipe  it off now and then, 'cause it's going to get dirty.