Regarding the discussion about sound deadening the rear cabin wall. I have the TRD duals and they have a slightly annoying drone at highway cruising speed. I ordered some sound absorbing foam from McMaster-Carr over the net, and it worked perfect for this application. The part number is 5692T49 and the description is "Flat Sound Absorbing Foam, Adhesive Back, Skinned Surface, 1" thick, 54" wide." I ordered 2' at a cost of $18.02 which was just the right amount to do the back wall in the Tundra. I also ordered a sheet of the generic Dyna-Mat (part number 9709T19, polymetric mastic damping sheet, adhesive backed, 32"x54") from McMaster-Carr. I removed those back vent covers and covered the outside of the covers with cut-to-fit strips of the sound damping sheet. I did not cover up the holes in the vents. I plan on using the rest of the polymetric dampening sheet in other areas later.
I couldn't ask for better results. This mod resulted in a significant reduction in road noise and I can barely hear those TRD duals now while cruising.
That 1" sound absorbing foam is the trick. Per the catalog specifications, it has a noise reduction coefficient (NRC) of 0.80, which is the best I could find in a 1" thick or less absorber (an NRC of 1.00 is the best you can get and typically only available in foams >3" thick). Rather than just dampen the sound waves, sound absorbers are designed to convert the sound waves to heat energy, thereby eliminating them (with an NRC of 0.80, it eliminates 80% of the sound waves that strike it).
The website for McMaster-Carr is
www.mcmaster.com
Alan