Not too much of a problem as I'd always wanted to replace the system as the range of the old one was quite limited.
I had bought a keyless entry system from Aliexpress since 2 years ago and had no good reason to install it until a week ago when it rained so heavily that it shorted my siren +12V wire to ground and rendered the old keyless entry system useless.
http://www.aliexpress.com/item/Free-Shipping-CF904-Keyless-Entry-Car-Alarm-System-With-Flip-Key-And-Logo-Trunk-open-function/493509728.html
This was the system that was purchased and the instruction manual looks like this.
Honestly, even for an electrical engineer, the instructions were quite useless...
Then I found help,at Peachparts forums, where many classic Mercedes owners, contribute technically to helping other owners, in this thread.