Car alarm that goes off when in a certain distance

[hiding@xxxxxxxxxx]

  Hi,
Hope someone can help with this.
I would like to be able to set up a alarm on a car. It would only go
off if the car got in certain distance of some point, say 100 feet.

In other words if the car drove up to where I work my alarm would
sound that the car is nearby.

I looked a bit for proximity alarms and really couldn't find much what
I was looking for. Any websites, adivce you could provide would help.
Thank you