Well, last spring I had to bypass the heater in my '84 Euro 300TD because the severe window fogging became very hazardous. But the last time I priced one, a heater-core for my Euro TD's manual heat-AC unit, was somewhere around $400+, and I really dreaded having to yank the entire dash and heat-AC system. Then I read on one of the forums that it's NLA anyway.
So I'm considering plumbing in one of the spare Chevy Cavalier heater-cores I've been saving for many years. The heater-cores themselves are perfectly fine and were only replaced because GM heat-shrunk extension pipes onto the cores to connect them through the firewall. The tubes cracked off, but left nipples perfectly suited to connect with rubber heater hoses. Running longer hoses wasn't possible on the Cavalier, so those cores were replaced with new all-aluminum ones with the aluminum extension tubes factory-welded on.
My theory is that perhaps I can hang one of the Chevy cores under the passenger dash and run a duct into the recirculating vent of my heat-AC box. As much as I'm able, I plan on as little butchering as possible. The worst would be the two extra holes I'll need in the passenger firewall for the new hoses. I'm sure I'll also need to block the vent flap to stay in the recirculate position. And possibly go into the fresh-air intake below the wipers and restrict that as well. I may still need to allow a bit of fresh air-flow to cool the blower resistor.
I wish I'd heard back from the person I PMed on one of the Forums that claimed he rigged a Ford heater-core in his W123.
Any thoughts, comments, gasps of horror?

Happy Motoring, Mark

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DrDKW