I'm baffled... I guess the only solution is to stick the multimeter in the connector and see what gets juice... I definitely don't have a grey/yellow option. When I figure it out I'll post the solution! Thanks for taking the time to look for an answer!
You need to be very careful when reading the electrical diagram information.
The picture you posted only shows the 5 wire connector (e.g. section 1) in your E4 right taillamp.
When I re-ordered the pin numbers, as shown in electrical diagram, to the same left to right orientation in your picture, I realized that it was for your right taillamp, which has different color coding than the E3 left taillamp, that as an example, I mentioned in post #4.
Specifically.......
1. None of the wiring in your picture applies to the E4e3 reverse lamp.
2. As shown in the electrical diagram, you'll find the E4e3 lamp connections in a connector (e.g. section 2), which should have a brown (i.e. ground) and yellow/white ( i.e. hot) wires.