ML270 Auto Shifter query

[sappers]

Hi, I have a 2004 ML270 and one evening last week the shifter refused to go into Park. It remained in Drive or Neutral but nothing doing with Park. I disconnected the battery overnight to see if this made a difference and also as the key was stuck in the ignition. Next morning I reconnected battery (no change) and set about removing the shifter. I swapped the shifter for a known good one (2002) and managed to get into Park and remove the key. The ML270 was now stuck in Park. I can override the shifter using a pencil down the emergency hole and everything works properly; gears change, low ratio works, high ratio works, shift changes are normal etc. There are no codes logged using MB2 code reader and no warning lights on dashboard.
Brake light switch I hear you all saying.....it's not on the brake pedal, so where is it?
I've checked the brake lights after swapping the shifter and none were working. It turned out the fuse had blown due to the bulb in the third brake light melting and short circuiting. I have checked all fuses and relays that had anything to do with brakes or transmission etc.
What I need is some advice re. the brake servo and master cylinder. The brake servo has two electrical connections and the master cylinder gas two connections. Do any of these control the transmission interlock?
Looking for a definitive answer as google and forums all seem to contradict each other.

If you are still reading this, then thanks for your patience.