Hey I have one of these, and I've battled with the DRL system and the automatic headlights system, in the process of trying to install an HID projector system into the car.
First of all, it's really easy to disable the DRL's: It's a little two wire, wiring harness plug, just in front of the fire-wall on the passenger side, under the hood. Unplug it, and the DRL's are disabled, everything else works normally.
Second part is a bit tricky, because I'm not sure if all Solara's have automatic headlight activation. Mine has it, the sensor is on the top of the passenger side of the dash, just behind the windshield. I wasn't able to find any easy way to disable this system, other than defeating the dash sensor. (Never bothered to try, because it was ok with me if the lights came on at night, or they will go off when I'm parked, and my parking brake is activated... even a little bit)