Note I normally drive with the headlights on, but I want the ability to turn them off or flash them (like in my old 240), which I can’t with the stock switch.

Some switches apparently have a setscrew to turn DRL feature off. This post applies to those switches that don't have that screw.

The part no on mine is 3515978. It is the same switch as on my '90 740. There are 5 contacts on the back of the switch. It appears #5 is the new wire that powers headlights whenever engine is running.

Carefully separate the switch halves. The moving part of the switch has two dumbbell shaped rollers that make the connections with different segments inside the switch. The contact labeled 5 on the back of the switch is the DRL connection. Just remove the indicated roller and pin 5 is disconnected. See photo.

Now I get the side marker lights always on when engine running, but the headlights only when switch is on.
I’ve been running for a week like this and have found no problems.

I’m happier now.