I've spent days researching vehicle tracking solutions. GPS tracking units require a monthly service charge and LoJack isn't transferable to another vehicle. Both cost well into the $100's. I decided to use a spare cell phone. This solution won't provide real-time tracking or any of the other great features offered by GPS or LoJack solutions.
What it does:
In the event my car is stolen, I can provide the police with the cell phone and IMEI number. They can use this information to locate the vehicle on the cellular network. With the optional microphone and auto-answer function, they can possibly call my phone and record the thieves chatting.
What you'll need:
Cell phone - Preferably one with an auto-answer function. I'm using an old Tmobile Motorola A630. Cost - $0
Prepaid SIM card - I'm using Tmobile's Pay As You Go. Cost - $10 activation plus $25 for 130 minutes. While writing this post I discovered the minutes expire after 90 days. All the carriers have some sort of expiration on the prepaid minutes. Tmobile has a Pay By The Day option which costs $1 every day the phone sends or recieves a call plus 10 cents per minute. I'm going to call Tmobile and ask a few questions and i'll update the thread on which option I continue to use.
**Update - The Pay As You Go plan switches to yearly expiration once you hit $100. I've decided to refill the account every 3 months with $10 refills until I hit $100 and then I'll buy $10 refills once a year. At that rate, I'll spend $80 over 24 months and add 240 minutes to the account. I could also spend $75 upfront, but there's no need to. I want to test this out before putting more money into it.
Car Charger - Tmobile car charger that came with my phone. Cost - $0
Accessory Outlet - RadioShack 12VDC Outlet. Cost - $6.99
Optional:
Headset with microphone. Came with my phone. Cost - $0
Before installing I wrote down the phone's IMEI number and verified the auto-answer function worked. Put the phone on silent. My phone won't auto-answer with the phone on silent, so i had to choose the lowest volume and a short beep for all sounds. I also adjusted all the settings for maximum power savings.
**Update - To find the IMEI number, dial *#06#
I chose the left side of the trunk area to perform my install. Feel free to use your desired location.
The car charger reduces the 12VDC to 4.5VDC with a current rating of 0.5 AMPS. For now, I'm using the cargo-light wiring as a power source. With the door closed, it has 3VDC. When the door is opened, it has 12VDC. This lower voltage may not be good for the phone's battery life, so i'll probably end up running power from the dome-light wiring. I initially tried to tap the rear driver-side door sensing line as well as the rear hatch sensing line, but both interfered with keyless entry and my alarm system.
Start off by pulling out the lower left panel and choose a grounding point. I chose a bolt on the seat locking bracket. If you're going to use the optional headset, you'll see i ran the microphone to the seat belt area.
From the cargo-light wiring, tap the pink wire.
Plug everything in and verify it all works.
I used Velcro to secure the phone.
This is everything buttoned up and put back in place. Again, you'll notice the microphone near the seat belt. Unless you're looking, it's hard to notice it's there.
Hope you guys find this helpful. I'm writing this at 1am so I may have forgotten something. I'll update the thread when i periodically test the system and work out any kinks. I welcome any questions or recommendations.
Update: 4-27-09
I was out of town for the last month. The cell phone is going strong. I've had a few random numbers call the phone, but they only used 6 minutes. The 3VDC seems sufficient in keeping the cell phone battery charged. I wont be routing a 12V wire back there. I left with the car's battery voltage at 12.6 and when i returned voltage was at 12.4. Very little power draw.
Anyway, cool project, great documentation/info - thanks for sharing!
I have a Tidewater Blue GD3 5spd! 
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