If you have the YD25 diesel it's possibly the oil cooler/heat exchanger that's split internally allowing oil to pass into the cooling system. It's a circular item bolted to the side of the oil filter housing under the filter. It's not too difficult to replace and not that expensive. The easiest way to check it is to remove the cooler and use compressed air. Block one coolant entry and apply compressed air to the other. Use a regulator as you only want about 15 psi. Immerse in a bucket of water and if that is causing the problem you'll see a stream of air bubbles coming out of the centre which means there's a path for the oil and coolant to mix.
You may also have coolant in the engine oil. Oil is less dense than water so the H2O will settle to the bottom of the sump.
