[VetteVet]

The L wire runs from the dash cluster charging light through the firewall to the alternator plug. If you reroute it to a key on 12 volt hot source the alternator should charge but the light will stay on all the time unless you take it out of the circuit or take the bulb out of the holder. Keep in mind that the light is a resistance for the alternator exciter circuit and it also helps protect the regulator from over voltage. You might need to wire in a resistor in series with the new L wire to avoid this. After a certain year I think it was 94, the resistor was included inside the alternator. I will try to find my diagram for the 90 year and see if it shows the path of the charging light L wire.

Both of the wires are 12 volt positive and do not run directly to any ground.
If you have not determined that the L wire is bad, you should take the alternator and have it tested to be sure it does not have a bad regulator inside.