Helm panel light switch de-energiezes the port tachometer

[ddfour1]

I have a fault I can't source.  When I turn my helm panel light switch on, my port tach goes dead.  When I turn the panel lights off the port tach immediately re-energizes.  I checked the wires for both the tach and the light switch and nothing looks pinched or otherwise compromised.  Is there a relay that might have failed?  Any other ideas?  2003 3500 Open. Thanks