^ The "smart" alternator is a relatively new feature. European models have had it on their Navaras since the early 16th century, we got it here in Australia in about 2011. It's no surprise that the average person in an auto parts store doesn't know about it unless they have a specific, special (most likely personal because they had the issue themselves) interest in smart alternators.
The DC-DC charger that you're choosing to use will actually charge the battery better than an isolator AND it will allow you to add a solar panel at a later stage. However, there's a couple of tricks in using it with a smart alternator (so you might consider disengaging that green wire anyway).
First, if you DON'T also have a solar panel, you need to connect the car's power to the SOLAR input, not the normal input. This is because the solar input will self-regulate and better manage the incoming power if the voltage is lower.
If you DO have a solar panel, you'll need to use a SPDT relay in the solar input side. You'd connect it like this: pins 85 and 87 together and to the 12V input from the car. Pin 86 to earth. Pin 87a to the solar panel and pin 30 to the solar input of the D250S. 87a is "normally on" (connected to pin 30) so when there's no 12V from the car, the solar panel is connected to the D250S. When the car is turned on, the relay turns on, connects pin 87 (which is car power) to pin 30 (D250S) charging the battery from the car and NOT the solar panel.
If you remove the green wire, you don't have to muck about at all and you'll get a better charge rate.
