Cruise switch is a snap. Took me maybe 20 minutes to complete the exchange. The part was like $28.00 from Fastlane or PartsShop (can't recall which) and was genuine MB.
Yes you have to pull the air bag and steering wheel. Probably safer to disconnect the battery, but after reading several tech opinions I opted to try with disconnecting this time with no problem. When storing the air bag unit, be sure to store in a cool place with the metal side down (I store mine in the trunk of the car if it is in the garage).
The bag is attached with two #30 torx bolts, but you can use a 4mm allen in a pinch, just use care so as not to strip the heads. The center bolt for the wheel is a 10mm and will be tighter than ol' dick's hat band. Use a short extension and a solid breaker bar, best to have a helper to hold the wheel with counter force. DO NOT attempt to use the wheel lock to hold counter force as you will likely break it.
Now slip the wheel off. Under the wheel you will find a plate with several spring loaded contacts sticking up. The set at one o'clock are for the air bag and the set closer to the hub are for the horn. On the face of the plate are three small phillips head screws, remove and store. The combination switch is under this plate and the cruise switch is mounted to the combo switch by a slip fit. The combo switch is mounted to the steering shaft by three more (larger) phillips head machine screws, remove and store these. Also note that the air bag contact are mounted to the plate directly under the wheel, and have a blue cable that runs alongside the cruise cable. These cables pass through a plastic fitting mounted to the combo switch, this fitting just slips out when the two side are pressed inward toward one another.
You will have to remove the steering column cover, which is easy on the telescoping column. Start with the column extended and once the combo switch is loosened, retract the column and the column cover should just come off. Not sure how you'd wrestle one off a fixed column.
Not sure on the 124's, but on a 126 you have to also remove the driver's side lower dash and knee bolster just to access the plug connection for the cruise switch (unless you have real small hands).
Install the new switch to the combo switch, connect the plug and reinstall all your pieces. Be sure you get an audible click when reconnecting the air bag (yellow connector on newer units, red on older). Now all that is left is to test the system.
I think that's about it. Hope this helps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.