Hi John,
Please tell us what bulb you are using and if you are using the original light sockets. The common 6V bulb is the 1154 style. Do a SEARCH of old posts using the terms +light +bulb +41 for more info. Get the story on members updating their bulbs to LED.
Here is a sample of a previous discussion:
41 Tail Lights.
Light levels can not be improved with a wire harness in poor condition or poorly grounded. I have replaced my harness and have excellent light from my 1154 6V style lamp bulbs.
Good luck, Mike