If you have access to the plugs in the back of the switch, a bit of detective work can identify the function of each connection. You'll need jumper wires, a pad of paper and pencil. I've done this with my mirrors, speaker system, and leveling jacks.
Here's a link to Velvac wiring diagrams, they are much newer than 1993, but perhaps use the same wire colors as your older system.
Library | Velvac[0]=field_resource_type%3A57