300tdi Auto ABS help please

My Japanese import 300tdi has recently thrown up an abs fault that appears to be brake light switch fault, diagnosed by bridging 5 & 15 on the OBD port and getting 2 - 6.
So, I replaced the switch with a brand new item and still have the same issue as before even after clearing the error. I have checked fuses, brake lights work fine (as they did before) and the interlock for the auto box in park works fine also.
Can anyone please give me any pointers as to what to look for now and/or how to test them?
Many thanks.

The brake switch has two contacts within one for lamps and interlock and the other for management, you have to get handy or gymnast and measure on the switch's plug in situ to see if you get 12V on the WR(white/red) wire coming from fuse F4 sattelite fusebox then you should get 12V on the WS(white/slate) with pedal completely depressed(ignition on), if yes get to the ABS ECU(behind LH side of fascia) and see if the 12V from switch gets to it's plug to pin 10(the same WS wire), that's the path for the switch input to ECU which according to the fault code is missing... if the feed is OK i',m affraid you'll need another ECU, see pin disposals face view

Thank you sierrafery for the comprehensive diagnosis, I will investigate at the weekend and hopefully find the solution. Presumably a replacement ABS ecu is plug and play unlike modern cars where they need programming?!

Let's hope it's just a bad contact on the input's path and you'll find it but if not i can tell you that on the 300tdi's the ABS ECU is just simple plug and play

I finally got round to doing the checks and all inputs are good so it looks like the ecu is goosed!
I'm kind of relieved in a way as I'll get one off a well known auction site and hopefully it will sort it. I'd rather this than a possible loom fault. Many thanks again sierrafery.