So...what exactly am I looking for online when I go to look for this oddball part (the hose that the red arrow is pointing to)? If you've seen my recent post regarding my (not so good) attempt at installing a manual heater bypass Pex valve in my '81 300D, I think the best thing to do would be to buy a second hose, check for proper fit, then cut the original and install the Pex valve somewhere in the middle. The second hose would be the "backup" newer hose to go to (if needed) once I've cut the original and attempted to splice in the Pex valve.

So far, locally, I've not gotten much help in finding some type of universal 5/8" bendable hose that won't kink, ergo my part ordering idea.

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I re-plumbed our 2002 T&C minivan using 5/8" silicone heater hose. That was while converting to Evans Waterless coolant. Some turns were fairly tight but the hose didn't kink. That was my better stock w/ ~4 nylon plies which likes to stay round. Some 2-ply I used did want to kink in tight bends. Don't get me started on the Ace garden hose I spent $$$ for a few years ago that now kinks everywhere, cutting off the flow, but that is a home problem, back to cars.

I also used silicone hose in my two 300Ds' for the run from engine to passenger-side tube, replacing the evil and useless "aux water pump" in the process. That is a gradual curve and might be a better place to install your manual cut-off valve. One 1982-85 300D guy installed a 1986+ mono-valve there (extended wires), since they are cheaper and better. For that location, regular 5/8" heater hose might not work because one side is a bit larger, but the silicone hose is more flexible so easily slid over it. Also, silicone should last the life of your car. If the hose you show ever becomes unavailable, there is much pre-formed silicone heater hose marketed to tuners, though would have to search to find one w/ the needed bends, and also usually sold in a $60 set. Hot rodders often cut a radiator hose and rotate on a metal splice to realize the bends they need, which is another future option, and applies to you if you put the valve there. At least that hose is standard 5/8" ID.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Appreciate the info, Bill. I've been "experimenting" with various locations of the Pex valve today, but still want to test further before I make a decision on where to try the install. My experimentation and decision will be posted in the next couple of days on my original "Monovalve" post. Regardless of where you choose to splice in, there really is no "straight hose" to easily adapt a manual valve to. Practically all the ideas involve the cutting, splicing and rotation that you mention. Perhaps a good reason why I've yet to see a post anywhere on the internet describing a complete DIY of bypassing the monovalve/heater control, or even real good pictures of it.

Again, I am experimenting, and will let folks know where I will attack this problem in the next few days on my "Monovalve" post.