Need help finding drive thrue monitor

[wlltx]

One of our challenges has been the fact that we some times don't know when customers are at the window. The employees are cleaning for restocking etc....... So i am looking for something that will chime when a customer pulls up to either side of the window. I know radio shack has a door chime system but i don't know if this will work with our set up.

i have posted some pictures of our building on my personal website and if anyone know of a system that will help please post or pm me. thanks.

http://mycanon10d.com/ipw-web/gallery/Whole-Lotta-Latte

thanks in advance

Felix