The missing channel is the front/rear fader in the console, it's dirty. Some contact cleaner will usually fix it, but if not it's not that expensive. I can usually get mine to work pretty well anywhere but the center detent.

The 603 is a bit tight, but nothing like the 300 in a W115 chassis!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!