Follow the link below, labeled "How to replace a Lexus IS300 tail light"
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
yea, same size
Changing dash panel lights 1995 Mercury sable how do I do it
One or more of the brake lights or tail lights are burnt out.
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).
You can change the tail light for a 95 to 02 by changing the trunk LID by changing the lid it allows the light to fit.
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.
try changing the fuse to the tail lights as well.
There is a cosmetic panel on the back panel of the trunk. If you remove that, there are 4 bolts on each side, and the tail lights pop out. You cannot get to the bulbs from inside the trunk, they must be removed.
open tail gate.on the side inside box you will see two bolts.take them out and the tail light comes out
Access the trunk, lights are held on by bolts and connected by a wire harness behind the carpet.