no crank, no start, no comminication with ecm, tcm, (Merrcedes benz S class w221

Hello guys
I had problem with w221 petrol / 350 witch randomly shutting off . and when it goes off t, no crank, no start, no communication with ECM, TCM ( checked with diagnostic tools). actually, it can not recognize the gear situation. BUT SOMETIMES THE car starts without any problem in the engine or shifting.

the FAULT CODES ARE common in many modules s:
1-no CAN Communication with ECM
2- no can communication with TCM
3 - no Can communication with ABC module
I also have a common fault code in a lot of low voltage fault code in almost all units,
when I traced the low voltage problem, I find out rear battery drain so fast. A fully charged battery AFTER SWITCH the car in acc or ignition will drain in few minutes to 8 volts.

more about it:
1- it has also no can communication with abs and esp
2- I have 2 main feed 12 v on ecu and tcm and grand as well.
3- I have 2.3- 2.5 v can in all of my can pols
4- I also have no access to wiring diagram of the the car , so I would appreciate if any body could give me this wiring diagram .
please help me with this problem.

What year is this vehicle?

Have you checked resistance on the CAN? I believe, it should be 60 ohms.

I suspect, if you find your battery drain, you will find your communication problem. The battery drain will be easier to diagnose, too.

Do you have and amp clamp?

first thing I would do is verify all good powers and grounds going to modules in question you check canbus resistance too