Just tracking a mobile phone, without any special application running on the phone, and without making a data (GPRS) connection is not possibly for normal members of the public. (It is technically possible, but is only able to be done by mobile carriers and even then, only when handling requests from law enforcement authorities). All those bits you see in the movies where they trace a handset are works of fiction.
The only way you could track mobile handsets would involve them reporting their location through GPRS/EDGE or some other cellular data service.
An adaptation of Google Latitude may be possible, but, again, would require all your drivers installing the software, providing permission for you to see their location updates and then ongoing data costs to connect to the Latitude servers and update their locations.
There are others out there who can provide transponders which you can install into your vehicles which would do all of this for you, but, of course, they would charge a premium for that.