I installed a spotlight that I bought from a guy who was parting out a police car.

The install was a bit tricky. Having to drill such a large and permanent hole in the van was the scariest part. It was probably the scariest install that I've ever done on a vehicle.
Basically I looked at the A pillar on my Grand Marquis and compared it to the Sienna. They looked to be about the same angle. From there I figured that it would fit. I looked on Craigslist and found this one for $25. They retail around $200. I guesstimated where the hole should be drilled and it came out good. Drilling was tricky and I was sad because I broke a couple of my smaller drill bits while making screw holes.
I had to notch the door with a hack saw. Then I cleaned it up with my Dremel.
The spotlights themselves have a really cool design. The bulb housing can be rotated infinitely because the electricity is carried through the shaft via a rod that is part of the moving assembly.