According to partznet, the DRL system includes a different flasher relay ($31.65 at partznet). According to the shop manul there is also a light sensor mounted at the top of the dash ($105.55 at partznet also used for the auto on/off head lamps). It's unclear if a non DRL equipped system has the programming in the ECU to support DRL and/or if the harness connector for the sensor is in the dash.

Based on my recent experience inside the driver's side dash, I'd say there's a good chance the connector is there. All of the connectors for the (unused on my sr5) dash switches were sitting in there. So my guess is if you add these components, you can get DRL and auto on/off head lamps. Of course to get the headlamp thing to work , you'd also need to upgrade the dimmer switch assy ($88.08 at partznet) or add your own switch ($1 at radio shack) to short pins 16 and 12 on the associated 17 pin connector (this would be easy to do).

So it could be $130 to add DRL (or less if the sensor is already there) and then the cost of a switch and some splice connectors to add auto on/off headlights. I may have to add this to my future mod list. ;)

UPDATE: You don't need the sensor for DRLs. Just the flasher.