Here's a great source for LEDs, it's an Ebay store named Chi Wing LED product shop. here's the address to it: http://stores.ebay.com/Chi-Wing-LED-...enameZl2QQtZkm
I assume you will want to use Red LEDs, which operate on 1.7V, not the 3V - 3.5V that many other colored LEDs operate on. I have used groups of LED's to replace interior lights, turned out very nicely. I would recommend using several LEDs in series to get a group that takes between 6 & 8V, then calculate the resistor you would need to operate it off 14V not 12V, as they will overcurrent when your engine is running if you base it on 12V; probably wouldn't harm them immidiately, but it will decrease their 11.5 year life!
When deciding how many LEDs to group in series, keep two things in mind: 1) if you make the group voltage too close to 14V, they may not even work with the motor off or while starting the motor. 2) if you make the group voltage too low, you will need larger power handling resistors to dissipate the extra voltage.
In case you don't know how to find the right resistor value, it is:
(Supply Volts - LED Group Volts) / LED Current = Resistor Needed
NOTE: LED Current is in Amps, NOT Milli-amps, so it usually will be 0.02; also, the resistance is in Ohms.
Make sure you use the proper power capacity resistor also, as too small will overheat, and in severe cases could become a fire hazard. The power dissipated (in Watts) by the resistor will be:
LED Current * (Supply Volts - LED Group Volts) = Power Dissipated
Hope this helps ya out!
As for how to make your own light, I used 1/4 inch plexiglass for my interior dome light. I make a paper pattern, then use a sharp object to etch it into the plexi. Find a drill bit the same dia. as your LEDs and a drill press. You hafta go slow, but drill out holes for each LED, then press them into the holes.. If you have the right sized bit, you should have to press them in tightly. If you want to diffuse the light pattern of the LEDs, just sand blast the front of the assembly after you insert the LEDs. Finally, get your 15-30Watt soldering gun / pencil and solder the LEDs into your groups with a resistor for each group (see previous post). Then connect all the + & - sides of the groups and solder on your red and black wires (or pink & purple, doen't really matter as long as ya remember what's what
For a taillight, i'd probably not bother defusing the light.. just be sure to aim them straight back.