Trace the whole route
Inspect the cover channel and drain inlet, then trace the hose to its outlet. Look for debris, crushed sections, unsupported loops, sharp bends, and a low point that holds water. Test with a small amount of water only after confirming the route and keeping it away from the cabin.
The Bing export included drain-hose and drain-hole questions with one impression each. That supports a practical diagnostic, not a universal hose layout.
Limits
No cover, bed, hose, seal, or rain event was tested. Exact routing and drilling rules remain product-specific.
Sources and related reading
See Also
If you want to move from general advice into actual product choices, start with Does Every Tonneau Cover Need Drain Holes?, Tonneau Cover Insulation: What It Can and Cannot Change, and Roof Rack Strap and Tie-Down Basics: What to Know Before You Hit the Road.
For a wider picture after the basics, Soft vs Hard Tonneau Cover Water Resistance: Which Offers Better Protection from the Rain? and Extang Trifecta 2.0 Tonneau Cover Review: Fit, Features, and Trade-Offs are the next places to read.