Thanks nachi11744, there is no visiblity into the heater box to see the heater core or lines, even through the bottom vents when they're open. I can't believe how well packed everything is under the dash for such a big car - I can now believe they say that MB designed the W108 car around the heater unit !! ... Still a brilliant product of engineering though.

As a last resort prior to pulling everything out, I'm going to remove the windshield wipers and cowl cover, and leaf filters today to see or reach any of the heater lines.

Cheers,

Ysmael