Hi, folks. I finally found the info on wiring your own fault code reader -- here it is (I hope it makes sense). BTW, the original source is from The STAR (the magazine of the MBCA), May/June '95.
Use a ''red'' wire for the 12v.power connection, through a 470-ohm resistor in line with an LED, to 3pt. juncture (i.e., to 2 more wires) :
1) to a yellow wire with a 1-amp fuse to a signal probe.
2) to a black or brown wire, through a n-o (normally open) momentary switch, to a ground connection.
Or, you can buy p/n 126-540-00-44, $40.
The article says to press sw. for 2-4 sec., and count...
Okay, that's the info. Thanks to all for your input.