They seem to me to usually be parked somewhere near the spur road that leads into the central T1-3 area. Might be a car park / tea shop there....

One thing to look out for. The times where I have a ride seem to be booked and confirmed, only to have the driver cancel, have been preceded by the driver taking quite a while to get away from that area. Whether they're getting shown better fares and cherry picking, or queues are too huge and they're giving up I don't know as I am not an Uber expert. But it can happen.

So once you have your booking confirmed, it's worth keeping half an eye on it to make sure the driver is truly en route.

Once you see them progressing around the perimeter road (assuming that's where they're coming from and in my experience it almost always is) then you're probably fine.

Of course if you're trying to finesse the timings to minimise waiting in arrivals that will probably be either while you're going through immigration (if you have no checked bags) or in reclaim (if you do).
If the former then it's a bit harder to keep an eye on your phone of course.

Screnshot of google maps posted to show where I mean as I probably described it badly.

edit - others have confirmed it's a remote car park there while I was busy attaching

HTH