This should be fairly easy.
The headlights are powered from a relay that is activated by the switch. All you have to do is activate the relay. One way to do this would be to tap into one of the wires powering the interior lights and run it to the correct pins on the headlight relay. Thus, when the interior lights turn on, the headlight relay will be switched on and then the highlights should turn on.
This is very similar to the hack that I posted to control the engine compartment fan using a manual switch (
Engine Compartment Fan Manual Switch).
What I'd have to check is what type of relay the headlights use (standard Type 53 or something else) to figure out which pins to jumper and if the 2003 S is wired close to the same as the 97-99 base that I have - but the principle should be the same even if the exact details are different.
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