Heater Core Flush?

They simply refer to it as a heater hose assy. It's a fairly common failure. The good thing about plastic coolant fittings is that they don't corrode like aluminum fittings; the bad thing is that over time, they get brittle and break. The only reason I could see that they use the plastic fitting is that it is a really tight bend from the heater core fitting at the firewall to the pipe on the firewall, so it would be easier to make by using a plastic fitting. At least they are not too expensive on the Frontier; the ones on the Pathfinders w/ rear heat are around $175. As far as flushing the coolant system, there is very little need to do so. Today's coolants are long-life, so usually a drain and fill at the intervals recommended by Nissan serve well enough. If you have a 2007, you're still using Nissan green coolant, so the first interval would be at 90,000 miles and every 60,000 miles, afterwards, if I'm not mistaken (you can check your owner's manual to be sure). If you use concentrated anti-freeze, mix 50/50 with distilled water (not tap water). As mentioned, if you still have the original radiator and have an automatic transmission, it would be a good time to replace the radiator due to the trans cooler issues with them.