They are physically interchangable! For the most part you will see no difference. The 3157 is a little brighter than the 3057 when the parking lights are on. For brake light/turn signal flash, the brightness is the same. It would be best to see what the bulb on the other side is and use the same thing. Better still to replace all the brake lights now. If one has failed, the others are probably close to failing too.
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