I'm glad this finally paid off for you. However, don't think that just because this is the first call from someone who mentioned your decal, that it is the first time it is working. For three years you have been driving around and people have been seeing it, giving you name recognition. Perhaps then when they see your name or brochure somewhere, they already feel somewhat familiar with you.
When we watch those Burger King commercials, we don't jump up and run to BK. But, when we are driving down the road and we see a Burger King and Joe's Hamburger Joint, we are going to stop at BK because we have seen their commercials and we're familiar with them. If someone said "What made you stop in here today?", you'd most likely say that it was the first sign you noticed, or was easy to access, but make no mistake about it, all those commercials you were seeing played a big part in your choice.
So! I think your decal has probably been working all along!