Make Sure your ABS ECU is connected properly, on my RHD 1987 it's on the passenger side, behind the battery, near the OVP Over Voltage Protection relay. That's another thing to check - if this is not in, or the fuse has blown, your ABS light will come on. From there, you'll want to check sensors etc. My dads shop has a nifty diagnostic device you plug in, and you then spin the wheels to check if the sensors are working.. there's a load of other stuff it checks too. I'm not sure if its useable on the 190 system, I presume so as its a standard Bosch sys.
I needed a new ECU recently, cost me £75.00 from a Mercedes Salvage co.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black