I'm not sure how you look for a power draw with a test light. That is only going to tell you that you have a complete circuit when you touch it from what you are testing to ground. You really need an ammeter or a DMM that measures current. Am I right??? It's early and I haven't had my first Dew of the morning yet.
A testlight in series with the negative cable disconnected from the battery works. The reason is all the radio memories and other stuff draw very little current, usually less than 30ma. The testlight needs more than 30 ma to light up, so that's why it works. If you look really close you might see it glowing very dim,, but a very bright testlight means you have a drain that is high enough to run the testlight, and that is too much.
You can use a multi-meter, but it has to be properly setup and then you have to know what you are looking for. Generally anything over 70ma is bad. The testlight is so easy to hook up, and give s a good non-technical indication, that it's what I recommend most people use.