It really is a very simple retrofit , and the system is very similar on W201 , W124 and W126 .
All my W126 cars had cruise , but I did retrofit it to a W 124 once .
There are basically three or four components , depending on the car .
The steering column stalk
The cruise control amplifier - the amplifiers are basically all the same , but there is a coding plug attached which optimises the system to different vehicles , although it will still work with the wrong one .
The throttle actuator - these differ from one engine to another , so you need to get the right one for your engine - on the M103 it sits right at the front .
Some cars require a Hall effect sensor fitted onto the back of the speedometer ; cars with electronic speedos , like W126 , don't need this .
You also need the wiring loom to connect these parts together .
The best way is to find a donor car with the same engine as yours , this will be either a W126 300SE , or a W124 300E/CE/TE in your case , and strip all the parts from it yourself .
It takes a bit of time to follow the wiring loom and unpick it without cutting , but it is possible . It links the speedometer , control stalk and the amplifier above the drivers footwell , and runs to the throttle actuator at the front of the engine . If you take your time you can unpick it from the donor car without damage and fit back into your own car .
Make sure to get all mounting brackets and extra bits of throttle linkage needed to fit and get it all working .
There aren't so many of these cars in breakers these days , but still plenty breaking for spares on eBay .
