You may have to perform surgery. It has been 4 years since I had the mirror apart, so I'm straining to remember. There is a steel disk w/ tabs that holds a spring under tension and the base to the arm. They may have forced this disk to slip, causing loss of tension.
If you do a search on my posts (username) and (mirror), you will find where I posted the procedures how to take the mirror apart and put back together. I made my 02 towing mirrors power/heated.
Let me know if you can't find it and I'll look for it.
Wiredawg