On the M104 that trim switch I believe is called the R17 CIS-E Resistance Trimming plug. It is usually located behind the battery, near the OVP on models it was equipped with. the info in the pdf is great info.
Here are the links to the PDF for the R16 and R17.
http://www.a-p-group.com/ashman/pdf/r16.pdf
http://www.a-p-group.com/ashman/pdf/r17.pdf
Let us know if anyone can make out how to retrofit the CIS-E resistance Trimming Plug or something custommade to work similar. I have a friend who builds car stereo amps, and I can have him make up a custom switch if needed, so I'm not worried about being able to get it done, but it would be nice to be able to wire that up in place of my fader switch!!! hehe
I would assume one has to make it hook into the R16 resistor plug the R16 resistor plugs into.
Alon