The vanagon cooling system is oversized for the zetec. Add to this that the water pump moves a lot more coolant than the boxer, low restriction direct paths out of the head and into the send line, the lower temp t-stat and what do we get? Well it's very hard to overheat the thing for starters... folks can take the system into the worst ambients imaginable with an underperforming radiator and low coolant and still not overheat... and the engine likes the slightly lower oil temps from a longevity standpoint but we sacrifice heat output with that configuration (good in summer, not in winter). But changing the balance is really easy.
We need to simply restrict the flow in the radiator send line to balance and increase flow in the core lines, but only in wintertime when it's needed. Even a slight restriction sends a lot more coolant to the heater cores which is what we want. Currently the path of least resistance is the radiator send line... and we don't want to occlude the opening all the time either (for instance if we machined the outlet smaller) since between the two pro/cons: 1) lowering cooling system performance 2) having great heat output without a balancing mechanism needed, one can kill an engine in the wrong conditions, one just gives people cold feet until they balance it.
So the technique we've been using is to use a hose clamp with a piece of sliced hose wrapped around it (so it can't bite into the line) and then make the diameter of the radiator send hose off the t-stat outlet 1/4"-1/2" smaller. People have asked "well why not include a valve" almost without fail, the answer is simple. Such a valve will be expensive (large, high heat), and what will you get in return? Two more hose connections and a minimum of three more failure points. Keep it simple! You can also change the balance without opening the system, and be done in a couple minutes. Also, what about the old cardboard trick? Well the does lower cooling system performance and raises coolant temps a bit to increase output, but it does nothing to address the real issue, which is balancing flow into the heater core lines... sure you can get more heat output with less higher temp coolant, but you can get just as much heat output with more lower temp coolant flowing and not increase your (cylinder) head temps.
Just like math, there are plenty of ways to accomplish the same thing. This method has the following attributes: