How do I work out the pin configuration of this crossfader with a multimeter?
The crossfader is a variable resistor, so you need to find which pins are the ends (inputs) and which pin is the wiper (output). The biggest resistance will be between the two inputs, and this won't change as you move the slider. The resistance between an input and the output will vary as you move the slider, and each input's resistance will move in opposite directions.

Your part looks like a Behringer CFM-2, but I couldn't find a datasheet.

Based on your comments, here's my proposed pinout:







simulate this circuit &ndash; Schematic created using CircuitLab