Hooking up light flash

Alrighty, just wondering what is the easy way to hook up headlights to flash with alarm. Obviously I could just hook a wire directly into the back of the light itself, but there must be a wire under the dash I can use instead, trouble is, how do I find it? Is it a case of tracing it back from the light switch on the steering column (which is almost impossible due to it being hidden on most cars) or is there an easy way? I think the relay for it is under the bonnet correct? So thats not much better than going straight to the lights....how do you pro`s do it? Ive hooked up to the indicator lights no worries, although the first time I wondered why both indicators went on at once...didnt isolate each side with realys :)

Cheers

- Dan

What kind of car might help

Just in general. I know it will be a different color wire for each type of car, but is there a general rule of thumb that you fellas use when hooking up lights? (ie, do you search for it under the dash, hook it straight to the light relay etc...)

Cheers

- Dan