Clear hidden data

Currently building a site for a car rental company. I have created a two-step process, with the first one only containing a hidden field to capture the page title, then a button sending the visitor to the next step, to fill in some required info. The hidden field actually captures the car make and model the visitor is interested in.

The process works fine the first time the field is filled with data. However, if the user does not submit the second step and decides to browse another (any other) vehicle, that hidden field is not cleared and always contains the first vehicle, until the second step is submitted.

How can I have that field cleared and filled with fresh data every time the page is loaded?

Here’s the field that causes my headache:
[dynamichidden car "CF7_get_post_var key='title'"]

NOTE: The site is still under construction, so please pardon the mess.

The page I need help with: [log in to see the link]