Good lord, *please* do not go to Best Buy. I had a close friend who worked there and we watched a few installations. He does his own installations, and said that he wouldn't trust Best Buy with the installation of a AA battery in a flashlight, let alone an audio system. I have had very good experience with Tweeter, Etc. in Boston, but am not sure if they're out in LA. I would definitely not go to any "superstore" (Best Buy, Circuit City, lord, even Wal-mart does installations at some stores now, etc.), but would find a specialized car audio/security place. You'll pay more, but it will get done right, and a lot of places will stand behind their work 100%. Crutchfield can be great for parts, but remember this: only do it yourself if you know *exactly* what you're doing. I put the stereo, speakers, and harness in my car, but have never done an amp, and plan on taking Hugo's new toy to a shop to have it put in. It sounds like you're keeping the speakers and the rest of the stuff in the Explorer, but do you know what the back end of the OEM head unit looks like, and what the situation is with line-in/out? You may have to look into what Dearing said about a 2-way switch...not sure, but good luck!
Festus
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Festus