Toyota 2000 Avalon left signal light working intermittently

SOURCE: Left turn signal does not blink properly.

More than likely it is going to be your clock spring behind the steering wheel.

SOURCE: left rear turn signal light on my 1999 chev. s10 is not working

any increase in bink speed is direcly related to a bulb being burt out.

SOURCE: dash lights go off intermittently

I'm no expert, I recently purchased a 2003 vibe. I noticed the automatic headlight's sensors can be super-sensitive, about an hour hours before dusk they'll kick my headlights on (when the sun is still shining!), or sometimes when I go into an underpass, and then sometimes they'll realize it's light enough and turn themselves off. This would cause your dash lights to go to the 'headlights on' mode and turn the headlamp signal on, the intensity is controlled by the dimmer/dome light control on the left side of the dash. Check to see if the dimmer is down too far, as this could cause the lights to dim when the automatic headlights turn on. It's also possible that the dimmer is not working, thus no power is getting to the dash lights through the dimmer. Try testing it by starting the car in daylight, then manually turn on the headlights, then adjust the dimmer. Like I say, I'm no expert, just guessing because I've noticed it happen similarly on my Vibe, but my dimmer was at half-way, as I like it for night driving. Someday, I'd like to disable the automatic sensors and DRL.