There is a full transcript of Rush Limbaugh's shows (not the edited ones the Limbaugh site shows) that you can access through a link at BuzzFlash.com. Advertisers are briefly listed in between segments. I was reading one of his transcripts and discovered the Guitar Center ad there.

I don't really understand the problem Kim C has with my letting people know what Guitar Center is doing, and what they can do if they don't want to support a company that supports Rush Limbaugh. No one HAS to boycott Guitar Center. I don't listen to Rush Limbaugh anyway, I never would, so that point is meaningless. But people who might want him off the air can use a boycott, and I'm just letting people know. I didn't know until a few days ago that they support him.