So I've searched, without success.

Is there a way to have the headlamp to with on when the wipers are active?

My ML and Explorer has that feature, its a nice safety feature that I got used to.

Can that be activated with TechStream? or is the GX platform just too old for that type of feature?

Tried to play light sensitivity in the setting on the head unit, but it seams that when its raining it is not dark enough for auto headlamp to switch on.

 

No it is not possible Natively. With that been said, you can always take the wipe on signal as the trigger to feed into light control ECU to turn on the headlights.

 

I assumed (most likely wrongly) that it doesn't do that because of the DRLs. My '09 Grand Cherokee would turn the headlights on with the wipers but it didn't have DRLs.

 

The other cars I have also have DRL, but that does not engage rear lights, additionally the DRL is only a LED strip, not really headlamp intensity but they will switch on full lights when the wiper is active.
But its a valid point as there are different rules around the world

That's a way, but not sure I'm up for that, yet