This is an old post. So hopefully I understand what you’re asking.
We aren’t talking about the same thing when I say “High impedance input”. I am talking about the input to the relay being “high impedance”. If you connect a voltage to it, it will turn on, but not pull any power. The low side drive output is not at all high impedance. Impedance is sort of like resistance. It would work the same if I said High Resistance (but I have never heard it called that).
Your volt meter is a great example of a high impedance input. You can put red on 12V and black on 0V and there won’t be any current flowing through your meter (assuming you don’t have it in amp mode).
The key is, if we connect that output to your relay’s input, is it going to pull a lot of current, or is it just going to measure the voltage? More info about the relay would help. Unless you know what you’re doing already.
I think the fan output (low side) also has an led and a resistor to 12V. Which may be good enough for a high impedance input on your relay. If not, a resistor in the 1kOhm-10kOhm would be fine. There isn’t a perfect answer here. It isn’t doing anything precise.