Posted for "Eric M. Jones" <emjones@charter.net>:

Ian,

Here's the skinny--

With the lamp on and your handy-dandy VOM set to current (A), measure the current in series with the lamp.
[probably something like 0.20 A]
Now, with the 12V, you have all the information needed to do what you want.
The lamp wattage is V x A watts [In this example 12 X 0.20=2.4W]. The present lamp resistance is V/A ohms [In this example 12/0.20=60 ohms].  (This is only approximate since the lamp resistance changes with current, but it's good enough for military work.) You can double or triple the series resistance and see how you like it. The resistor wattage for final installation is A X A X R. [In this example 0.20 X 0.20 X 60 ohms=2.4 W minimum].

As a final check, look at the lamp at night, and make sure the resistor is not too hot to touch.

Email me off-list if you have any questions.

Regards,
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge MA 01550-2705
Phone (508) 764-2072
Email: emjones@charter.net