Like David said before...
I added the wiring for the headlight washers (in case I go Xenon) and added rear headrests a couple of years ago. You can buy a complete headrest retrofit set at the MB dealer that consists of the wiring, vacuum lines (to connect at the headlight unit or vacuum block under the hood), switch, foldable headrest holders and you will have to order headrests and a different rear deck in the proper interior color. Adding the headrests and wiring all parts up to the front is rather easy and can be done within a day (adding wires in a W202 is a lot easier as in a W124).
If you want me to, I can send you some pics and additional information about the rear headrests.
To add the headlight wipers, you can also order a complete retrofitting set at the MB dealer that consists of wiper arms, wiper blades, wiper motors, relay, additional water pump and necessary hoses and you probably have to order a separate wiring harness. Pretty easy add-on as well.
You can check my web site (below) to see what else you want to add to your car...I would go for the power rear sun shade as well when you decide to add rear head rests.
Dorsey Lay...I completely forgot about the pics I promised you; I am sorry!!!
with a minor 600+k