trigger on DELETE

Hello everyone,

I'm trying to define a trigger that copies the row to be deleted into
another table (which is the inventory_audit table) before it does the
delete from the original table (which is the inventory table).

CREATE FUNCTION inv_audit_mod () RETURNS OPAQUE AS '
    BEGIN

        NEW.user_aud := current_user;
        NEW.mod_time := ''NOW'';

        INSERT INTO inventory_audit
        SELECT * FROM inventory WHERE id=NEW.id;

        RETURN NEW;
    END;
' LANGUAGE 'plpgsql';

 CREATE TRIGGER inv_audit_mod BEFORE
        DELETE ON inventory
        FOR EACH ROW EXECUTE PROCEDURE inv_audit_mod();

Ok, the function works only with a trigger that is defined as ON INSERT
OR UPDATE.  If I try to define a trigger for ON DELETE and then delete a
row from the table, there is nothing in the 'NEW' variable to return.  I
get an error message.  If I define the function to return NULL, 0, or
nothing, then it comes up with a type mis-match error.  Is there anyone
who can help?  Thanks.