I believe you do usually need a relay for that function.
I know I had to buy an additional one for my car and it was either for the flashing of the lights or the turning on of the interior lights.
I didn't install mine though, I'm patient, but not that patient.
You might venture on over to the forums at
www.the12volt.com
It's about the most comprehensive site that I've ever come across dealing with automotive electronics.
Here's the specific pagge that you're probably looking for.
http://www.the12volt.com/relays/page4.asp
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