The current limiting resistors arent needed, but why not, they are cheap. And I would recommend to put an opamp, something like a LM358 (you can get non-duals too, or even quads) and run it in buffer mode. Even if the input is huge, it can only output the rail which should be 5v. Extra precaution there.
As for using the existing temp sensor, Depends how it is read. If it relies on resistance and you hook up a huge resistor to it, you throw off all the readings (if you intend to keep the OEM working correctly as well). If it is current, then pulling current off of the line will again result in some weird readings. Voltage is what you hope it is. In which case you can run a buffer line off of it. For current, use a small resistor to get a voltage, and run through a buffer.
And now as I read further down I see you already experimented with this!
And you already have the answer, buffer opamp.
I like the idea of the univeral OEM sensor connector. Very nice. Took a while to stare at it to see what was what. Looks daunting with the wires everywhere at first