If there is a warning light on the dash of a IS300 Lexus it signals an issue with the car. The meaning of these lights can be found in the owners manual.
it is vehicle stability control
Follow the link below, labeled "How to replace a Lexus IS300 tail light"
Lexus IS300 3rd brake light is changed from the back seat. Pry the light up and disconnect wiring. Replace bulb and put back together. Easy job
this is for the 01 model. it might also be the same for the 03, http://www.porkfist.com/repairs/auto/lexus-is300-taillight.html
remove bulb from in side the boot
To change the high mount rear brake light on a Lexus IS300, first, open the trunk and locate the access panel on the rear deck where the light is housed. Remove the panel by unscrewing or unclipping it, then disconnect the wiring harness from the old bulb. Replace the old bulb with a new one, reconnect the harness, and reattach the access panel. Finally, test the light to ensure it’s working properly.
Bulbs can be replaced you have to get bulbs type 9006 (hb4)
small ones are 24 the large are 194
You must remove the plastic "buttons" holding the trunk lining (flathead screw driver). Remove enough to get lining out of the way to have access to the back of the tail light assembly.
check all outside lights to insure they are not blown
2003 Lexus IS300 tail light bulb replacement: Purchase the correct replacement tail light bulb(s) first, before disassembling the vehicle. See sources and related links below for replacement bulb information. Then consult your owner's manual for the tail light bulb replacement procedure. As simple as it sounds, the owner's manual is the best place to start for bulb replacement instructions and illustrations. You may want to consider replacing both left and right at the same time in order to keep brightness and color equal (optional).