For those who are looking for the CD changer fiber optic cable on a 1999 SL500. Many thanks to the forum members who helped me with this.
The cable is on the left side of the trunk running from behind the antenna to the taillight assembly. It's easier to find if you completey remove that piece of trunk trim. The cable is attached to a bigger dark cable and is brown (I think, I am color blind) with a yellow twisting stripe. The end of that wire is the fiber optic connector for the CD changer. The power plug for the CD changer is bundled (taped up) with 3 other connectors: a coax-like connector, a 15 pin female connector and a 25 pin female connector that I've been told is for the phone.
I did not need to bring the car to the dealer for any programming of the head unit; I literally plugged it in and it worked. Mounting is another issue and I have simply attached it to the trunk "floor" with velcro. I'm told that skipping is a problem in general with my changer (MC3198 from eBay.) Good luck.