PL372

left tail light go on, but does not brighten whrn brake pedal is prushed

Gene01

The tail/brake light bulb usually has two separate filaments. Replace the bulb because one (the high-wattage one) is burned out.

PL372

First thing I did was change the bulb with another W5W.. Didn't help

ThChinaman

Was the bulb that you tested with a dual filament bulb? W5W bulbs are single filament.

PL372

Hmmm. Owner manual states it is W5W. I will pull the right tomorrow and see what it is

Gene01

Also, the dual-filament bulbs have one bayonet pin close to the end of the base, and one further up (nearer the glass envelope). This is to insure the correct polarity (brake = high wattage filament, tail = low wattage filament).
However, if you try hard enough, you can deform the socket enough to put the bulb in backwards.
If you don't try hard enough, you will not seat the bulb completely.