Here is one group's solution:
http://www.blind-eye.com/SCAP/index.html
That works well if you go with the presumption that the equator is through Barbarian Altanis, and the climate is semi-tropical to tropical.
Another place that might work if you envision the equator further south, past the southern edge of the Wilderlands map, would be just south of the Southern Reaches.
During a campaign I played in a while back, our judge placed the city in the mountains between the River Stillring and the River Severn; we never actually got there, though.
Another place that might work is on the Isle of the Blest, which has plenty of mountains and jungle-cloaked hills.
You could also put it on the border between Tarantis and Karak, in the Terad Range.
Or heck, even in the Valley of the Ancients; never know what might be found there.
It all depends, really, on where it would fit best for the needs of your campaign. That's the beauty of the Wilderlands. Make of it what you wish, and no one has the right to gainsay you.