Missing enable on +5v sec pwr rail AMD Firepro D500-B/Mac Pro late 2013

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Sub: Missing enable on +5v sec pwr rail AMD Firepro D500-B/Mac Pro late 2013

Ref:  1/ MacPro6,1 Late 2013/A1481 MD878LL/A (3.5 GHz 6 Core Xeon E5-1650v2)
            Gfx-B: APN: 661-7548 PPN: 820-3533-A AMD FirePro D500-B 3GB Vmem
         2/ Figure 1 Gfx-D500-B Front Facing
         3/ Figure 2 +5v supply (zoom-in)

Intro:
While troubleshooting a Mac Pro(6,1) referenced above I discovered the +5v secondary supply did not stand up on the D500-B graphics card as a result of a missing enable signal.  The +3.3v secondary rail was the only rail that did standup.  I tried to perform a logic/circuit trace with power applied on the pcb but found this to be high risk do to lack of accessibility while probing and thought reaching out to the technical community for technical direction would be a better approach. 

Diagnostics:
Instead of micro-troubleshooting I injected +3.3v dc on the enable pin through a 22k ohm
Resistor.  The +5v supply stood up.  Note that probing pin 10 for resistance resulted in 42k ohm and in diode mode 0.539v.  I attempted to trace pin 10 back to its’ source and this is where I need assistance.

Mahalo for your support