Bad ignition coil?
 

Please correct me if I'm wrong, but I should be able to get continuity between the negative post and the high-voltage post, right? I did get continuity between the negative and low-voltage post.

Depends on ign system.
Reg systems can be teid between pri/sec. where some ECU distribtorless systems [ coil overs /waste spark] use isolated pri/sec winding induction coils.
Which model/chassis are you testing.

W126 380SE Bosch ignition coil.

That should be a standard coil, so you should have a reading on the seconday .. however , make sure your meter is set on high ohms scale or it will not show [ will show as open circuit/no continuity if you are using a low ohms scale.]
The sec windings are very high ohms , campared to primary.

So, use a low scale across the small terminals and high scale across the high tension terminal and a low terminal.
See what you get then..

It is good meter practice to start on high scales and work down, then you can be assured of proper ranges and when working with voltage or Amps scales , you will not damage a meter.. Many newer style meters are auto ranging, but this is good to know anyway..