72 C20 - Wiring through firewall question

[jvictorv33]

Hey All -

Putting in new stereo/amp for my son... drilled through firewall for amp power cable (crutchfield kit). Snaked 8 gauge wire through supplied grommet and inserted. Current test shows everything is fine from battery to and through in-line fuse box and in the cable immediately pre-grommet pass through. Current test immediately past-grommet inside the cab shows no power.

Fire wall current force field? What am I doing wrong?

Thank you for the help.