I actually wasted 3 tailights to do this...good practice. I found a Dremel to be the best at cutting into the assembly...
To cut the LED's out without being visible from the outside, the opening will be almost the same size as the LED's... Just be careful and press/twist the assembly back into position gently.. If it came out, it'll go back in.
Sealing is vital, these LED's are on a silicon strip that if they get wet, that's it. Make sure you seal them tight!