Why you are not getting anything is because, you are not posting anything!!
You only have one
<input> and it is this one:
<input class="showInterest" type="submit" name="showInterest" id="showInterest" value="Show Interest" />
And, that is just a
submit button only, It does nothing by itself. It can only submit what is inside another
<input type=''> tag.
ex: if you have
<input type='text' name='carYear' />
You would be able to retrieve the value by saying
echo $_POST['carYear']; in your PHP page.
I don't see anything there. You can't post a plain text, no matter if it is inside your
<form></form> tags. You have to create inputs.
Ok, put this values scripts your HTML file,
<input type='text' name='carYear' value='' /> <input type='text' name='carMake' value='' /> <input type='text' name='carModel' value='' /> <input type='text' name='carColor' value='' />
and fill all the
value='' with the car details, and submit the form