That small bulb is a side marker light and is part of the tail light circuit. It is supposed to illuminate when the rest of the tail lights/parking lights are turned on (first click-stop).

It is really there to help illuminate the side of the lamp assembly for visibility and regulations. It eliminated the need for a separate rear side marker lamp found on the previous generation of trucks.

What bulbs are you using? Incandescent or LED?

My Owner's Manual says it is a 194 type bulb.

Check the bulb sockets as well as the connector for the tail light assembly for corrosion. Double check that both new bulbs are actually good.

 

 

Like old87yota says^^^
Pls see my signature, too.

 

Always good to check if this has a trailer light converter box they cause all types of light problems .

If I remember there is a Tail light harness plug behind the left tail light

 

If you get to the point of checking wiring, the tail light harness disconnects under the passenger side seat. You might need some extended leads to test the harness. From my experience, the end of the tail light harness is hacked up behind the tail lights in 4/5 trucks because someone tried to install some shotey trailer light kit made for a domestic and those kits almost always come with scotch locks. If your wiring is shot behind the tail lights, I make pig tails for them, if it's really bad I can make long pig tails, or the whole harness minus the cab connector (haven't source it yet).

 

 

 

I am glad it was something simple like bad bulbs.

I have seen a few people replace some expensive parts when the end the bulbs were bad. That's why I mentioned it.

 

YUP. Always start with the simple, the low-cost, before you even take things apart.