This explains how to build a cable to connect a SNES (or N64) to an AppleColor RGB (IIgs) monitor. You can use the information with other monitors as well, but they must be capable of a horizontal scan rate of 15.75 kHz (modern VGA monitors cannot scan this low.)
You will need two Multi Out cables, as they only have five connecting pins each. Using a flat blade screwdriver, open up both casings. Remove all five pins from one and insert them into positions 1,2,3,4 and 10 on the other. You may want to use a putty knife or similar instrument to wedge open the clamps holding the wire on each pin, to allow you to solder the new wire directly and keep things clean. Or use heat shrink tubing to keep the connection points insulated. Attach a 1k ohm potentiometer between pin 10 (+5V) and pin 3 (RGB Synch). You will have to adjust this (to about 300 ohms) to ensure a good stable image. On the Apple side, solder 220uf capacitors to the three color pins, negative side towards the DB-15. Without the capacitors the image will be dark and washed out. If you get small enough capacitors, they can fit inside the DB-15 housing. Solder the other connections to a few feet of eight conductor cable. Hook up the RCA audio to your stereo and the RGB to your monitor. If you want, you can run the audio through two unused pins on the DB-15, and connect the SNES, GS, monitor, and stereo to a switchbox.
SNES Multi Out AppleColor RGB DB-15 11 9 7 5 3 1 8 7 6 5 4 3 2 1 12 10 8 6 4 2 15 14 13 12 11 10 9 pin 1 (red video) +220 |( pin 2 (red video) pin 2 (grn video) +220 |( pin 5 (grn video) pin 3 (RGB synch) - pin 3 (comp synch) pin 4 (blu video) +220 |( pin 9 (blu video) pin 5 (GND) - pins 1, 6, 13 (RGB, RCA GND) pin 6 (GND) - RCA audio L/R GND pin 7 (SVHS Y) - no connection pin 8 (SVHS C) - no connection pin 9 (NTSC video) - RCA video pin 10 (+5 V) ~300 ohm no connection pin 11 (audio left) - RCA audio L pin 12 (audio right) - RCA audio R