I think on that model the oil light is a level light, not a low pressure light. On the Tundra 4.7, I think its at the base of the oil filter housing. But I dont know on a Corolla.
It's a low pressure switch for the light on the Corolla and it has nothing to do with oil level. Not sure where the switch is on the 03. It's not as simple as just tapping into the wires though.
You need to buy a gauge and a sending unit to accomplish what you want. You can either use the stock port where the oil pressure switch is and lose the light (or run a t type fitting space permitting) or use a sandwich adapter on the oil filter. I went with the adapter on my 99 Corolla when I installed the OP gauge.