Mr`revto9k

while changing the speakers today on my 01 s2k today. when I removed the passenger speaker, I noticed this white plastic thing inside the door frame.

Utah S2K

It appears to be a "Stabilizer" for the window. Used to provide alignment pressure. It's the only internal door part that looks close to it. Never seen it myself but looks like it snaps over an adjustment bracket towards the upper rear of the door.

Utah

Ref: 2001 Helm Honda Factory Service Manual Page 20-8 & 20-13 for anyone else to comment