On a lot of stock vehicles, the sidemarker, parking, and taillights are very dim, nearly useless. Especially on some older vehicles that have lenses that may have lost some translucency with age.

Luckily, there is a very easy and inexpensive way to fix this problem.

If you go into an autoparts store you may notice that a lot of bulbs look like the same sizes, even though they carry different names and numbers. Well, guess what - they ARE the same sizes, and they can be interchanged with each other, in a lot of cases. Stock units are often 1157, 2057, or 194 bulbs, depending on the size of the bulb. But, there are other bulbs that will fit with no modification with enhanced brightness (CP are candlepower):

2057 Low:   2 CP   5000 hour life
2057 High:   32 CP 1200 hour life
1157 Low:   3 CP   5000 hour life
1157 High:   32 CP 1200 hour life
2357 Low:   3 CP   5000 hour life
2357 High:   40 CP 400 hour life

All of these bulbs are interchangeable, and you'll notice that switching from a 2057 bulb to a 2357 bulb will increase light output 50% on the low filament (for parking and sidemarker lumenance) and about 30% on the high filament (for turn signalling and stop lights). There is a tradeoff in bulb life but 400 hours for a stop light will still last for years in normal conditions.
For smaller bulbs, like the 194s, these can be easily swapped out as well:

161:       1 CP   4000 hour life
158:        2 CP   3000 hour life
194:       2 CP   2500 hour life
168:       3 CP   1500 hour life
W5W:   4 CP   500 hour life

So for some vehicles, you can increase the brightness of these units by 400%. These will make sidemarkers more useful, parking lamps more useful, map and glovebox lights better, and even instrument panels more legible. I wouldn't recommend the W5W/168 bulbs for instrument panels, however, because of the labor involved in replacing instrument panel lights when they burn out. But especially W5W bulbs make fantastic reading lights.

There are other bulb sizes and types, comprehensive information on all of them can be found at www.sylvania.com.

GE has released Nighthawk versions of some of these bulbs but only released Nighthawk 194's and 2057's, which are the dimmest to begin with, so these aren't that great. Also avoid LED versions found on eBay as these are never as bright as stock and in turn signal applications, make the car think the bulb is burned out and will rapid-flash (because LEDs draw much less power than a regular bulb).