They don't exist *now*
The last of them were scrapped in the early 90s. I pulled the engine and gearbox from one in a scrapyard to fit to a neighbour's ice cream van.

It was a late-70s Transit, with a single row of "minibus" seats and rear windows, then a low bulkhead, then some 19" racking bolted into the floor. The roof aerial was still mounted, although all the cabling to whatever had been in the racks had been snipped off.
(gordonjcp, Fri 2 Dec 2011, 15:45, 1 reply)

I heard they were really just empty,
and they only had big aerials on the roof for effect.
( Moon Girl Technologies horrendous beanbag, Fri 2 Dec 2011, 15:54, closed)

I heard they were used to delivery mind-control contrail gas to the smaller airports.

( Dr. Shambolic je suis charlie, Fri 2 Dec 2011, 16:16, closed)

Wouldn't be in the least surprised
It would certainly be cheaper. Some were definitely real though ;-)

On that note, a certain local constabulary has a couple of magnetic self-adhesive signs that look like a camera window that they stick to the side of a perfectly ordinary police van then park up on a motorway flyover - *sometimes*. And sometimes it's the real camera van.
(gordonjcp, Fri 2 Dec 2011, 16:57, closed)