Speaking of Chrysler minivans, my 2002 T&C 3.8L had some plastic coolant fittings. One broke when I was fiddling with the heater hoses and I wondered "WTF is this for?". It was a small inline nipple and it broke for many owners. It simply added a slight restriction, perhaps to reduce noise or to slightly alter the proportioning of flow to my rear heater. I tossed it and no noise. If flow ratio was the purpose, why bother since you can just adjust the rear blower speed for more or less heat. There were also 90 deg plastic fittings which turned upward thru the floor just in front of the rear wheels (only if rear climate option). I tossed those too when I re-hosed with silicone hose everywhere (lasts forever). I just slightly bent the aluminum tubes to better match the turn. Might have also been a tee to send coolant rearward. If so, I replaced those with copper fittings.
But, not all plastic is bad. My 1985 M-B transmission has fairly large plastic parts inside, but nylon which seems to last for a long time even in hot oil. I recall some small nylon parts in Chrysler transmissions too. But, not sufficient for the stresses of the camshaft gear, which had nylon teeth in the 1960's for less noise, since those quickly wore (my 1965 Chrysler as I recall).
