Welcome to the Remote Starter Extender Project
This project got started  with the culmination of several things seeing several cellphone hacks over the years on Hack A Day, owning a car with a remote starter, having an office that is too far away from the parking lot to use the normal key fob and the ultimate motivator... winter weather!  Of course some people think you're crazy when you want a feature that just isn't "out of the box".  Those are not the kind of people I would be expecting to read this. ;-)  I figured I would document my little project and hopefully you enjoy it. 
- Aaron
The Problem...
While this normally isn't really a problem in most peoples books, it is a bit of a walk(I know others have it worse I'm sure...)  A little walking is good for you! When it's between March and October things are fine.  It's just between November and Februrary that I'm not happy. I hate the cold and for years I've dealt with walking out to a cold car and waiting for it to warm up and the windows to defrost just like everybody else.  Eventually I came to the conclusion that there has to be a better way!  Why can't I start my car from my desk and let it get warmed up without me sitting in it cursing old man winter?
First of all, this is the criteria I set for myself:
  • Must allow me to start my car from my office(several hundred feet away with numerous steel and concrete walls in the way)
  • Must not cut into the cars original wiring(I'm sure the dealer would frown upon this and likely void my warranty!)
  • Must provide some sort of security. I can't just have anyone starting my car after all!
  • Should be reliable
The Plan
I thought about this for quite a while.  My first thought was to buy a prepaid cell phone and wire it to a 555 timer circuits and a spare key fob for my car. 
I bought a cheap cellphone to play with for ~$12:
I figured the easiest way would be to tap into the phones pager motor.  When the phone rang it would trigger my circuit. (similar concept to your typical terrorists IED just without the mallicious intent)  I started playing with it and ran into a problem pretty quickly.  There was no apparent way to prevent other people from calling and starting my car.  It didn't let me set a vibrate profile for a specific user.  It was a all or nothing situation.  That seemed like a deal breaker to me.  While I'm sure there are several ways around this problem and my "Hack-Fu" isn't as strong as some. Not discuraged I did some googling.  Of course there were other comercial GSM remote car starters/alarms that I found but they all required me to cut into my cars wiring(not something I wanted to do)   I ran across the concept of a GSM controller used by vending machines.  The seemed to be overkill for my needs but still interesting.  Finally I found something that seemed to fit my needs exactly.  A comercial GSM switch made by http://www.gsm-auto.com.  It seemed to be exactly what I needed.  It provied 2 remote controlled relays, security based on the incoming telephone number and ran on 12v.  At $199 it's not cheap but certainly in line with some of the comercial car alarm/starters I found.  This was the way I was going to go.
It took a couple weeks to arrive on my door(they are appear to be based in Great Britain).  Here's what I got: 
Taking the cover off the unit:
The unit requires an active SIM card.  I used the one from the prepaid phone I bought earlier.( recommend T-mobile because the minutes don't expire for 3 months unlike other carriers in the US  A $10 refill card will give you 3 months of "remote start service")  Here I a temporarily used a motorcycle battery to program the switch.  The swicth is programmed by sending it text message commands.  I programmed it to fire relay 1 for 1 second, wait 1 second then fire relay 2 for 2 seconds.
While the remote switch alone wouldn't serve my purposes.  I needed a spare key fob for my car to sacrifice(Ebay ~$15)  If you look below you can see the circuit board removed from the key fob.
 This definately wasn't my best sodering job, but I added leads to the "lock" and "start" button contacts and used a little tape to secure them.  
I don't want to use this as a long term solution but taping the remotes original battery in place should work for now.  Eventually, I'll wire in a 12v to 3v converter(Ebay had these great little premade converter circuits for $5.49 shipped) 
Here I wired the "lock" button contacts to relay 1 and the "start" button contacts to relay 2:
I wanted to tuck the remote keyfob inside the GSM switches case but I was concerned that I might short circuit so I repurposed a plastic bag and wrapped it around the key fob board:
I'm planning on putting the unit in the center console of my car which has a cigarette lighter in it.(perfect for powering the switch)  I wired up a power cord using some heavy speaker wire and a replacement cigarette lighter adapter.
Here's everything wired before I put the case together. 
Everything assembled:
Installed the switch in my cars center console
Here's a quick video of the remote starter in action:

YouTube Video

1/4/2010 Update
On one of the colder evenings (about -6F) I tried to start my car before my train arrived and everything seemed fine when I called the GSM-AUTO unit(ie it hung up immediately)  However when I got to the station I found my car had not started.  I tried calling it again with no luck.  I used my regular key fob and the car started.  I'm speculating that the low temprature weakend the key fob battery wired to the GSM-AUTO unit so it couldn't send the signal to the car to start.  The next day car started with my phone as normal but of course it wasn't as cold.  As I mentioned above, I need to wire the fob board to the car's battery with a 12-3v converter.  It doesn't make much sense to have a remote starter that doesn't work in the cold...lol
1/27/2010 Update
Thanks for all the interest from the Hack-A-Day crowd!  When I put something together the first time you're always excited to see it work and sometimes you take a few shortcuts(ie taping a battery in instead of actually wiring it to a power supply).  In this case I had everything working for a month or so and it mysteriously stopped working.  The GSMAUTO would respond but not start the car.  Not surprised I took it apart and of course the battery had come loose from the key fob circuit board.  So I'm addicted to having an unlimited range remote start so it was time to fix it properly.
I removed the battery from the key fob and wired in the 12v DC converter I showed you earlier in the project. (The disconnected wires in the photo will be connected to the 12v power)
After some initial testing I found that the 12v converter wasn't dropping the voltage down to 3v as anticipated. It was only going down to around 7v.  This was preventing the key fob from working properly.  After some research on the converter, it seems that the converter cannot provide stable voltage if the current is below 5 mAh.  I added a 2.2K resistor between the coverters + out and ground.  Voila, I had stable voltage that I could drop down to 3v for the key fob!
Here's a picture of what it looked like before I put the case back on.  I put a small plastic bag around the converter as well.  I didn't need it shorting out against the GSM-AUTO.  Luckily everything fits inside the case and I'm back in business.  I shouldn't have any more issues with loose batteries.  Actually, I shouldn't have to open the case again unless I need to change the SIM card or reprogram the key fob to the car again.