I think the headlights from DRL's will suffice as well. However when those are on your rear lights are not. If there is a mist or fog then people coming up behind you may not see you.

Perhaps it's my age making me forget, but when I leave for work (in the dark) and I start my work SUV, my headlights come on (DRL's). I can't tell you the number of times I simply forget to turn on my lights and I'm going down the road with no rear lights on.

Yes, I admitted I drive an SUV for work. However I get a new one at the begining of the year and I'm about 80% sure they are actually going to consider my request for an Escape Hybrid. That's a big leap for a large company to go against standardized specifications.

 

I have to turn mine on in the rain. My Highlander does not have auto on, but auto off, so my lights stay on all the time.

 

Yes, the sensitivity CAN be adjusted by the dealer.

I had the service department set my headlight sensitivity to where it would come on at the earliest opportunity (dusk) and stay on until the latest chance (late dawn). (I believe that the setting is "LIGHT 2".)

References:
Owner's manual page 120, last paragraph.
INTRODUCTION Service Manual;
Page IN-31.
Light Control Table and Illustration 1 table.

This is something that you CANNOT do yourself, unless you have the "INTELLIGENT TESTER" to change the settings.

I forget where I downloaded the manual sections from (possibly ToyotaNation).

I hope this gives you to data you need.

 

As an FYI, the Camrys do have DRL on/off switch but some other Toyotas do not have DRL off only on (i.e. 2006 Sienna/4Runner).