What you need is a peanut butter flavored air filter to keep them away from the insulation. Was it the firewall or underhood? I wish I had a part number for you. Maybe try some rat traps baited with peanut butter.
Dave
2003 Silverado SS here's where my money went...Radix @ 9psi, Vortec IC tank, Oversized Heat exchanger, ASM LT Headers, Suncoast Ram Air hood, Airaid intake, Trailblazer Torque converter, Full billet servos, Zippy kit, cat-free, Flowmaster muffler, Gaylord Speedstur tonneau, 3/4 drop with Belltech spindles and leafs, Sirius conversion & MTX thunderform sub box. 05 e-fans coming soon!