When I had to replace one of my inner fenders on Greta, my TDI Jetta in 2008, I found out that all the pine needles that fell in by the windshield wipers eventually made it through to that cranny between the firewall and the inner fender. ... to the tune of about 1-2 pounds of pine needles and trapped dust.
Per side! Yes, as soon as I found out the bad, cracked inner fender had this, I went and changed the good side too. Around 4 pounds of extra weight trapped in where you couldn't see.
The good news - Not too much for rust, considering what WAS back there, for up to 8 years before it was found...
But thanks for the reminder, come spring, I'll tackle this again, and finish rustproofing the metal tangs where the inner fender bolts on again, to be thorough.