[Marco Tahtaras]

If I have this straight you are using a battery powered test light to check for continuity. If you have the clip hooked to ground and probe the switch contacts on the harness then the test light should come on showing continuity to ground as long as you have good light bulbs in place. The light will dim some as the bulbs act as a resister.

Whenever I've encountered the problem you describe in someone's car it has been a loose connection at either the terminal box or ammeter post. I've also heard via the forums that some fuse blocks fail due to loose rivets which would cause enough resistance to cause similar results.