Caveats: This hack worked great for my license plate illumination, but it will not work for brake light application since these are usually dual filament bulbs and it will not work for turn signal bulbs unless you also change your flasher to one that is compatible with LEDs. There are many sites that sell LED retrofit bulbs and flashers for older vehicles for some examples, search Ebay for "LED 1156" or "LED 1157" or "LED 3056" or "LED 3057" or "LED flasher".
Parts required: 4 white LEDs
Note: For my build, I used LEDs salvaged from another project, and most white LEDs should work or this license plate illumination hack, but before you purchase your LEDs you should check the brightness rating on the data sheet and verify that it is adequate for your application e.g. Digikey part # 160-1850-ND has a brightness rating of 17000mcd.
Tools required: gloves, safety glasses, pliers, wire cutters, soldering iron, hot-glue gun, multimeter, power supply (for testing)