I thought the car engine was off while they waited, and therefore the air conditioning was off, so he wanted the window down to keep cooler.

I thought he wanted it down so he could start filming right away if something happened. I remember when I first took a pic through a car window all I got was glare so I roll the window down when I want to take a pic of something.

Another possibility is that he wanted to look casual.

Rick was in fear and maybe Lou senced the smell of it, which he apparently didn't like.
I myself can't really distinguish the smell of fear, but I heard some people can.
Anyway thats what I was thinking when I watched that scene.

I had assumed it was so that if he had to film something out the window, he wouldn't have to pause to roll it down. In fact shortly after he does have him film out his window.

I thought Lou just wanted the murderers to be able to get a good look at Rick.

I thought maybe he wanted to be able to hear what was going on, but didn't want to be spotted by the criminals through his window. Maybe a glare would prevent him from being seen. Or he wanted rick to be ready to film at any pointalthough he would have told him to get a camera ready if that was the case.

That crossed my mind too!

I seriously thought he was going to shoot Rick in the head and wanted the window down so his brains would fly through the open window!

They didnt give a reason. I posted this in another thread, but I think the point made in that scene was that Lou didnt really care about how Rick felt. He had some reason for wanting the window down, even though Rick didnt, and he made him put it down. I think its just another small scene to show that Lou does what he wants and has no sympathy for anyone else.

Most drivers prefer more cool air coming in while driving or when in car to avoid going to sleep.
That's pretty much the reason why he was asking him to lower the window.