EDB (and others) here is why I bought Ceramic tint on my last car and why I'm holding off on the Passport:
There is a local tint shop and they had a display, which I will describe:
Imagine four panes of glass attached at the sides to form a box (with no top or bottom).
This box was mounted over a heat lamp bulb (one that gets VERY hot).
The four sides of the box rotate and so that each side of the box can be in front of the heat lamp bulb.
Two of the glass panes have super dark "Regular" tint, like "Limo" or 5%.
While the other two panes have a 50% & a 35% "Ceramic" tint over them.
I rotated the 4-panes around and the LIGHTEST "Ceramic" blocked MORE Heat than the DARKEST "Regular" tint.
It's great and worth the extra money and you can do the windshield as well, to block even more heat!
And if you want to block HEAT, then YES, installing it over the already dark tint of the Passport will do just that.
I live in So. Cal. near the beach and it rarely gets super hot, plus I can crack my windows, so I'm going to hold off at this time, but having owned Ceramic tint previously, I can attest to how much better itis than Regular tint...!
Hope that helps...