As the 36 Chevy man saidand I have been posting when this topic comes up, it is best to repaint the inside of taillamps, parking lamps, backup lamps in a gloss white. Also, on the dual filament lights (stop/tail, park/turn) use a 2357 bulb rather than an 1157. The taillamp portionas are the same but the stop/turn portion is mnay times brighter!
The final step to the cheapest and brightest lights possible is to thoroughly clean the lenses on the inside. Use a good cleaner, ammonia and water works well, or vinegar and water, an old toothbrush and lots of patience. Scrub and rinse just one area, then hold the lens up to the sun or a bright light and see what I mean!
Then reassemble the light and be sure to put on some shades before you fire them up, don't want to fry your corneas!