The horn and steering wheel controls have stopped working on my 2007 e350 4matic mercedes

SOURCE: SRS light stays on

This is most likely a wiring issue in the steering column.  I advise you not to attempt to repair this on your own unless you are an experienced mechanic.  If you do be very cautious as the airbag wiring runs through there and can be a very dangerous mistake.

SOURCE: steering wheel controls

Replace the clock spring, or check all the cabling and plugs first. If you are inexperienced take it to an auto electrician, otherwise do it yourself but disconnect the negative battery terminal for 2 minutes prior to opening the steering wheel to check the plugs, as not doing so could cause the airbag to go off. In addition you can also take out the airbag fuse. Check radio control plugs and close it up again and reconnect everything in reverse order. If that doesn't fix it, then have the clock spring replaced professionally, its too much mucking around if you don't know what you're doing.

SOURCE: Horn, Steering Wheel, Cruise Control

Replace the Clock spring. It is mounted behind steering wheel. It alows electrical connection while the wheel turns.

SOURCE: Would appreciate help for a Toyota Corolla 2007 non-working horn.

If the horn works with direct connection, and fuse, relay, and circuit breaker are all good, it's the switch in the steering wheel. If it is anything like the chrysler auto's, the switch and the air bag module is all one unit, can't replace just the switch, the whole thing, and it's about $700 to $800 for those units, at least it is here, and can only get them at a dealer.

SOURCE: Horn, cruise control, and radio controls on steering wheel do not work ?

The clockspring behind the steering wheel is possibly defective or unplugged. It is the one and only component that all of your symptoms have in common.