Marker Lights only 9.7 volts Help!

[Jamesb]

After installing backup camera with existing plug that came with, I noticed camera lost signal every time I applied brakes. After checking different voltages, I only have 9.7 volts on my rear marker lights, and when applying brakes the voltage dips below the camera standards, and drops out. So, checked ground on pin box, voltage and all connections in the cord junction box under pin box. All is good, perfect 12.1 volts. Checked all bulbs including tail light bulbs. Junction box starts at 12.1 volts, the two front marker lights is 10.9 volts, then rear marker lights go down to 9.6 volts. Removed all bulbs and still same results. I wish I knew how the wiring runs. Removed a couple marker lights to check wiring but really cannot get to the wiring much. What’s my next step? Sounds like a grounding issue, but pin box ground is perfect. FW always stored in temp controlled environment, practically new condition, with zero corrosion concerns. I appreciate any help. I’m stumped. TIA