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The vehicle is a 2003 Chevrolet Trailblazer. The low beams aren't working. High beams are. The fuse is perfectly fine (tested and swapped out). The bulbs are fine too.

A mechanic told me it could be the "body modulator". I've never heard of this before. Could this be the problem? If so, how hard is it to change out for a shade tree mechanic?

If not, what else could this be?

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As Brian and Nick said, I would check the stalk (switch). When this happened to me I found some loose screws in the switch; it was falling apart. The low beam contacts weren't connecting. Mind you this car was 26 years older than yours. – Igby Largeman Nov 28 '11 at 17:04

2 Answers

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If you can get a copy of the electrical diagrams, you can trace them through and start testing each piece involved in the circuit. I had a similar problem with a Mitsubishi once, and it ended up being a bad headlight stalk... The low beam portion had failed somehow without impairing the high beam or parking light functions!

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I'd get a multimeter and check though the circuit - as Brian says the point at which the current is switched is a common failure point - either in the switch or the relay if one is fitted. I've never heard of a body modulator and can't see how one would be fitted in a headlamp circuit!

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