best engine heater for an 82 w123 300d turbo?
 

So it looks like my car once had a block or anti freeze heater but now all that is left is the cord. What's the best way to add a heater for these cold days?

Was the factory block heater installed and then removed or did you have an aftermarket coolant hose style block heater installation that's been removed?
If installed, the factory block heater would be located between cylinder 3 and 4 on the right side of the engine (between the starter and the exhaust manifold).
They rarely fail, so you might just need to plug the cord into the block heater.

Here is a thread in installing the MB block heater in the block:
Definitive 240D Block Heater Thread - Mercedes-Benz Forum

Here's a thread on installing a coolant hose type of block heater:
Mercedes block heater install

Both methods work, the coolant hose block heater is easier to install.
Double (or even triple) glowing with new glow plugs and using synthetic diesel motor oil can also be helpful for starting in the cold.