Hello Hannah
NickZ is bob on with his suggestions. If you want to read a more expansive post that brings together all sorts of resources and angles for thinking, you might like to look at the longish first post in my ‘Show Us Yours!’ section? This is accessible via the home area and is where various members have posted about their vans. It’s my write-once, refer-to-often post on resources, links, etc.
Your 2006 might be towards the modern end where these resources and links are relevant but you won’t know until you take a look.
Also on my show us yours first page (below the pictures of the rear brake drums) is a fairly long post on cooling. Again, it’s written from the perspective of a H100 but I’d expect there to be general similarities between the H100s and 200s.
The cooling system in Hiaces is fairly complicated (I think), a fact added because of the shape of the vans and where things are.
I wouldn’t want to speculate what the issue might be but rather than remove the manifold it’s a lot easier to look from below (some panels might need to come off) or from further back (even if the seat has to come out).
It might depend on the ancillaries you have. Autos tend to have a transmission cooler built into the upright radiator with feed and return lines. Vans with rear heaters have piping that goes from near the thermostat housing back then across to the right then rearwards (and back).
Hope this helps and good luck!!
Hope it's something simple.