You're most welcome for the pix. Once you get that lower panel off, the rest is very straightforward. There are a number of screws holding the panel, most of which are evident. Do a search on this forum and you'll find a pix I posted a while back on where all the screws are. Removing this panel is one of the classic jobs on the W140 in that it will take you a fair amount of time the first time you do it... after that, its only a minute or two.
I think the odds are in your favor that its the switch and not the ABS pump. As Steve said, they're a known failure item on these cars. The good news is that they are inexpensive and easy to change. Ordinarily the folks (especially Steve) on this forum would recommend against simply guessing to replace a part, especially on a W140 car. You can go broke aiming in the dark. In this case, I think its entirely worth the small expense and the odds are pretty good you'll solve your problem. Be sure to extend the new switch plunger before you install it.
Besides, you'll have the "opportunity" to learn something about your car that you didn't know before.
Good luck. Let us know the results. Lots of folks here willing to help.