LowXtreme,
I used to work at Best Buy myself, as the install supervisor in Cambridge, Mass. I saw plenty of installers come and go, but the better ones seemed to leave to find a more ethical and less stressful place to work.
Anyway, though, you already have a great start. You work in this industry, you've done a little audio work, and you even installed a keyless entry.
By the way, a keyless entry install, and an alarm install, are identical in most ways. Think of an alarm as simply a keyless unit, with the addition of a siren, a shock sensor, a starter kill, and a way for the unit to sense that a door is being opened. Oh, and of course you'd like to hide the alarm and its wiring better than you would a keyless entry unit. But other than those differences, they're surprisingly similar.
The absolute first thing you should do is get all the reading material you can. Every alarm you sell comes with two books---- an owner's manual, and an install manual. The customer usually doesn't get the install manual, which means that your installer friends will have a few in their toolboxes. Also, all of those little DEI bypass pieces have instructions too. The 555L will include a little information about Passlock for you.
These manuals are not the last word by any means, but they'll get you through a basic no-frills install. There are LOTS of tricks you'll want to pick up later on that will make the car more difficult to steal, and there will be plenty of unique challenges not covered in the manual. Oh, and every install manual is a little different---read ALL of them you can find, even for brands you don't like.
Find yourself a friend who has an easy car like, say, an older Accord or Camry. Hopefully he wants a basic alarm----he wants remote entry, a siren, and his insurance discount, but doesn't care about high security----this is the kind of friend you need. I can assure you that your early installs will not be terribly theftproof, but start with simple installs on simple cars----once you get the basics down, then start picking up special tricks.
Also, many installers use a tech sheet that gives rough locations on the wires in the car, as well as any special circumstances. Best Buy sells DEI products, which means they use either Techsoft or Directwire. Have your installers print you up some sheets to get an idea of what's going on----print up a few popular cars, and also ask the installer to print up what he thinks are the most complex cars. Print up a 2002 Cadillac DeVille or Seville and see all the weird places DEI wants you to go for wires-----it's never as hard as DEI makes it out to be, but that's still not a quick car.
The MECP book can't hurt, but it only touches lightly on car alarm installs as I remember. It will help you with basics such as using a multimeter or test light, how to test a wire in a car for proper operation, etcetera. But any reading is good reading.
Can you find a way to get some training at Best Buy? Even if your installers are bad, that doesn't mean they can't teach you things---at the very least, you can remember and learn from their mistakes. If you can convince your install supervisor that you're able to do basic deck installs, he might have you help out during a busy period or when someone is out sick. When you see your DIST, tell him you're interested in installing, and back it up by telling him what experience you have.
One of the great things about those box stores is the astounding variety of cars and situations you see---there might be a fancy, complicated car getting remote start in one bay, and in the other bay, the installers are trying to figure out/rebuild the wiring of an old car that's had five different radios stolen from it......if you have a sharp memory (or a notebook), you can pick up a lot there.
You probably know that you'll be using relays and diodes a lot, and resistors sometimes too. There are good descriptions of each on this site; read them as carefully as you can. Don't worry if you don't understand all of it now; it'll come to you.
Hey, that's how I learned this business. I got into home audio as a little kid because of my father, owning my first set of true hi-fi stereo components at age 12. Around the same time, I installed my first car stereo--a shaft-mount Audiovox and a pair of speakers in my cousin's '70 Nova. The installed turned out to be reliable, too. I joined my friends as a car stereo salesman at Circuit City. Shortly thereafter, the manager realized that I was a terrible salesman but had good technical knowledge, and brought me out back as an installer, where I caught on pretty quickly on the audio side of things.
After a few months, I did a few easy alarms with a seasoned installer alongside me at nearly every step. One night, I told my mother I needed to borrow her '90 Accord for something, and took it down to the shop after closing time. It took me a good four hours or more, but I removed a broken old alarm, and did my very first remote start install all by myself: A DEI 551R with keyless entry, and an extra relay to make the parking lights work. I drove home that night with a great feeling of accomplishment, as well as a Mother's Day present! That was just over four years ago, and it still works great too.
Sorry for the long story, but hopefully you get the idea. If you have the ambition and interest, a few easy cars and a few good friends who don't mind a couple of mistakes, and good websites like this one, you can do it.