So I thought this update to the first post would be better as it covers the full solution.

My E-Class 350 but I believe would be similar in other models started to shutdown my radio and then proceed to do 80% of all other systems.

The issue only starts once the ignition is fully on
The car also runs with no issues so I believe the Fuel Pump short is likely to be an old uncleared fault. But this might not be true.

The SRS alert came on and was permanent on my dash. (Think this is the case till it clears)

Normally its always the Comand system that shuts off first. Then the Presafe warning comes up on the dash and the rest of the systems fail to work.

Airscarf
Heated Seat
Vented Seat
Parktronic
Steering wheel controls
Heating system on minimal working
Intelligent light system
High beam does nothing
ODB2 often fails
Heated Rear windows and mirrors

Car had NO running issues and the following still worked:

AirCap
Roof
Windows
Dash Dials
MPG, Milometer still seems to calculate
Heating does run and adjust just low blowing
Mirrors Adjust and fold
Memory Seats and Steering Wheel

The problem got to the point that it rarely came back once failed, although I still got it that it can run for 30 mins+ no issues.

The Dash showed incorrect voltages 15v and as low as 7.5 and the car was charging fine at 14.66

The car showed NUMOROUS Faults about low voltage.


SRS - Supplemental restraint system (N2/10) (MB:2079011900)
Fault Explanation: : Fault: The squib for pyrofuse 'On-board electrical network line' has excessively low resistance.

PARK - Parking system (N62) (MB: 2129011500)
Fault Code: A10A00
Fault Explanation: Fault: The power supply in the system is too low. - No sub type information system undervoltage - no sub type information

ZAN - Audio/COMAND display (A40/8) (MB: 2129010500)
Fault Code: 00A10A
Fault Explanation: Fault: The power supply in the system is too low.
Fault Code: 00953D
Fault Explanation: Fault: Signal 'Low voltage differential' has an open circuit.
These are just a Few of the codes I got. I did many diagnostics but finally took it to CommandOnline for investigation. Typically the issue NEVER happened while it was in for diag.....<Angry> but they still did a full wiring check and we spoke through thoughts. As the system was shutting down and not cutting out the F32 Relay was excluded from being the fault.

This put it to either the Rear Sam or the Negative battery lead. Due to cost and ease and assuming the rear sam was working as it should as it was telling equipment to shut down and thinking it was low on voltage I bought a Negative cable (with sensor) and replaced it. (10 mins max. Mostly finding your spanners!)

Once replaced instantly the Voltage read 14.7 on the dash not 15 and varied correctly on start up to the volt meter. I have driven it now for about 8-10 hours with not one failure. I hope this article helps some people if they get this fault.

Thanks