Here's a handy indoor/outdoor thermometer which I purchased at Radio Shack and installed on the dashboard. It's indicating 78.7 degrees inside, and 67.5 degrees outdoors.
I initially tried mounting the thermometer on the blank rectangular panel at the top of the photo, to the left of the stereo. It looked good there. However, I found that the stereo put out noticable heat while operating, distorting the readings. The lightweight thermometer mounts with double-sticky tape, so it could be removed, repositioned and remounted fairly easily.
The thermometer has min/max memory, so it can keep track of the overnight low, or tell you how much you cooked the interior of the van while you left it locked up on a hot parking lot in the blazing sun.
It's powered by a tiny button battery inside the unit, so there's no connection to the vehicle's wiring system. However, it needs a wire for the outside temperature sensor. The wire exits the thermometer at the upper left side, where it disappears into the crack between the panels, and goes back inside the dash at the lower left corner of the dummy panel that's just above the "78.7" (two panels to the left of the "BRAKE ABS" indicator light).
I had to figure out a place to mount the sensor. It should be out of direct sunlight, away from the heat of the engine compartment, able to get a good flow of fresh air, and easy to run an unobtrusive wire to. I decided to mount the temperature sensor on the luggage rack above the passenger's head, as seen below. This photo was taken from inside the open pop-top, looking out the open front screen, looking down and toward the right.
The sensor itself is a tiny sort of cone-shaped device about 1/2 inch long, and perhaps 1/8 inch in diameter at the widest point, where it's connected to a wire. In order to shield it from the sunlight, I glued it inside a little PVC plumbing tee. I then attached the plumbing tee to the inside of the luggage rack using drywall screws. The wire goes down under the luggage rack, back in through the rain gutter underneath the pop top, above the top of the passenger door, down the passenger side pillar at the side of the windshield, behind the dashboard, where it connects to the thermometer. It's all hidden behind trim panels; the above photo shows all of the wire that's exposed, from the bottom of the plumbing tee to the floor of the luggage rack. I put quick disconnects in the wiring at both ends, so I can take apart the dashboard or remove the luggage rack if I ever need to.
The outdoor sensor does overheat a bit, say five degrees typically, if I'm parked in direct sunlight. But once I start driving, there's enough airflow through the plumbing tee to make the sensor agree almost perfectly with the temperature displays on most of the banks I drive by.