Just noticed this right now on my car. Bulb direction/install matters a lot, for even lighting! Fixed it right up like so:
FACT: These are dual-filiment bulbs. (17 in the diagram) So they have 2 'spring things' that light up inside the bulb (filiments). One FAT (strong/stop) on a straight wire pair, closer to the base, and one THIN (night/on) spring toward the head of the bulb on a curved wire pair. Take a look at the photo closely, and your bulbs too.
1. In order to get EVEN lgiht output, you have to make sure that when your lights are on (Night time, headlights and all) the THIN filiments inside the bulb light up on all 4 rear lights. These are a bit weaker, for constant night light.
2. When you press the brake, the FAT filiment should light up, brighter/intense during braking. (I used a brick to hold the brake down when I checked the rear lights. (Have your headlights off for this, helps to see)
Make sure that the straight pair FAT filiment inside the bulb face the same way as the click-thing in the bulb-mount (6 on diagram).
To test, follow the above... Dont just pop them in and hope it lights up even, the lights look soooo much more evenly lit when you do this. Sorry for my hectic description. Hope this helps.
$35? these are $4.95 bulbs Long Life from SYLVANIA has the LL after the model number.