1984 was the first year for the factory alarm, I believe. The 81-84 SD does not have a vacuum actuator in the driver's door, only a switch to activate the vacuum pump. Because of this, an electric actuator is piggy-backed onto the opening rod to work the door lock (and this switch) on the driver door. I did my 1984 300SD this way and it's worked great for a couple years now.
Maybe you could splice a vacuum line into the system and run it to a 1985+ actuator/switch in the drivers door and then hook up the keyless entry to the pump as others have demonstrated on the post 85 MB's, but it seems like more work.
The absolute best DIY (with full 52 pictures) on how to DIY for the 81-84 system and the 85+ system ( 21 pictures) is Thomas Pindelski's subscription site (~$30/yr.)
http://www.pindelski.com/
I'm not advertising his site; no affiliation etc., etc,.. but it IS good.