Well, I took the old switch out and am ready to get a new one. Called my local dealer $36.00 plus tax, online at Europarts direct $23.75 Genuine MB part. So while I'm waiting I decided to do a little more testing of the condition. So I do the following.
I removed the switch, connected the wires back up and taped the plunger part way closed.
I then started the car and let it run, No diagnostics as suspected as this is like first starting the car and not touching the brake, all is fine. I let the car run for a while and all is fine.
I then applied the brake and held it down for about 30-45 seconds, It then gives me the 3 diagnostics as before even though the plunger is fixed in the "no brake applied " position. Would this still indicate the switch or is there really an ABS malfunction?
Your test is flukey. The systems work in concert, so you're pressurizing the braking system and alerting BAS and ABS modules while the computer is not getting a signal that the brake pedal has been pressed...keep messing around and maybe you'll be successful in triggering a hard code that you'll have to pay the dealer to clear. (That's the other reason I suggested getting it at the dealer, then you get it and change it instead of being tempted to muck around with it while waiting for a part to arrive.)
BOTTOM LINE: 99% of the time when you get the three errors together it is the brake light switch. Whether 18 or 25 or 32 dollars, all are cheaper than running full SDS, are they not? (And a roll of tape and a multimeter cannot replace SDS, either.)
So your choices are simple.
A: Replace the switch with a genuine part and see if the issue is resolved.
B: Go to the dealer (or an indy with up to date SDS) and pay them to run diagnostics. (The caveat here is that there are plenty of tales of folks who did just this, and after spending a grand or more to replace myriad sensors, it turned out to be the brake light switch.)
However, there is no other "testing" you can do that will result in a magical Harry Potter moment. Leave the poor thing alone until the switch comes.
If it fixes it, great. If it doesn't, you may have exacerbated the problem with your attempts, but in any event it's time for expensive diagnostics.