Suggestions for modifying the trim in your car:

1. Run down to the local junkyard and get a spare trim panel, so you can restore the car to factory condition should you ever sell/trade-in. This also allows you to leave you car intact while you spend the time necessary to do it right. I think they charged me $15 for mine (it was significantly more going through the local Nissan dealer's parts dept.)

2. Very carefully measure how much you need to cut, mark it with a pencil (which easily comes off). Then use a dremel or similar tool to cut slightly inside where your mark is. You can always go wider. You can't go narrower.

3. Angle your cuts slightly so that they are narrower at the face of the trim panel than they are in the back.

4. Get some flat and round files and smooth out your cuts so that they are perfectly straight and round at the corners.

5. Make a small sanding block by wrapping a some fine sand paper around a small block of wood. Then proceed to spend an insanely boring few hours sanding the cuts perfectly smooth (remembering to keep the angle on the cut.) Recommend that this be done in front of the TV.

6. Try it out in your car and repeat steps 4-5 until you have the desired fit.

That's what worked for me. YMMV. I helped a friend with his before I did mine, and thus avoided some of the pitfalls he made. So with two of these under my belt, that's the sum of my knowledge gained from experience.