If you need a "hard" switch that cuts power instantly when the key turns, check under the dash.
Fuse #26 (10A) —labelled as TRL B/U, SEO, or "Special Equipment" / USB. 2004-silverado-switched-power-source
Here are the best, verified locations to find ignition-switched power on your '04 Silverado. 1. The Best Spot: Passenger Side Fuse Panel (F26) If you need a "hard" switch that cuts