I think on the tach, someone has told you about the sending unit like the older (early 80's) 300SD Diesels had. I don't believe your 300E has it. The tach signal comes from the TDC sensor, there is a small bus near the ignition control unit for this purpose.
Since you don't seem to know where the brushes are, I think we can assume they didn't get transferred over from the old switch. They press into the front of the combo switch, they are right on top, as the brushes of course contact the slip ring on the back of the steering wheel. The bottom of the brushes have a metal tab that lock into a pair of contacts on the combo switch.
You should be able to buy them at the same place you bought the combo switch---dealer, FastLane, take your pick, probably not a generic parts house like NAPA or AutoZone or one of those places. I think you'll be able to get the brushes installed without removing the whole switch. I'd also double-check the installation of the Airbag brushes, this may be why you have a flickering SRS light.
ps You'll have to buy 2 of the horn brushes, they are sold individually