When my headlights are on, there is a yellow icon of a car in the dash with three lines coming off each rear corner. In the area where the door open light would be. I dont have a manual, what does this icon mean?

the car is a 99 GS 300

 

Probably one of your tail light or brake light bulbs has blown. Have an assistant check while you brake, or if you don't have any friends handy, just reverse upto a sheet glass window and look at the reflection in your rear view mirror while you brake/switch lights on/off etc. I'm not sure what bulbs this warning covers so it could be reversing lights, turn signals, fogs, brake or tail.