Need GPS tracking referral

we have a small fleet of six sprinters, and we cover the a large area, the whole state of Florida. Can anyone recommend a gps tracking unit, we don't want the employees to know we have installed them. Thanks in advance.

They are used alot in the HVAC/R biz, have a look here. http://hvac-talk.com/vbb/search.php?searchid=12140432