Scrit for pop up like "Thickbox" when customer leaving checkout

We have a web shop sporttema dot se written i php. We want to do a survey on customers that go to our checkout and then leaves without buying. If the customer go further to the "Thanks for your order page" the survey will not be shown. The survey is in a specific URL you will get, from a company helping us to collect tha data. If the customer leaves the checkout or go back to the shop (not to the "adjust shoppingbag page") the survey will pop up in something like a thickbox (like then clicking on an image in a web shop, it gets bigger and the rest of the page is little darker) The customer must have the choice to answer the survey or "no thanks" and go further where they want. They must always have the option to leave. When survey is answered, it must be an close down button. Write the script that we can add in the shop to get it working.