Good point, I think it depends on the characteristic of the resistor you’re using when it is working at RF frequency (up to 3GHz), if you’re using a small (0402 or 0603) SMT 10K resistor, I’m pretty sure it will not impact antenna at all because the resistor will behave consistently as as a pure resistor, meaning no inductance or capacitance characteristic at RF frequency, and given the RF impedance of antenna and cable is 50 ohm typically, the impact of a 10K pure resistor should be negligible.
I used to work as a RF engineer, and did impedance matching work for mobile phone RF transceiver and PA, at that time the mobile phone antenna are external and we often add a shunt inductor to protect PA from ESD damage, so I don’t see any risk to adding a small 10K resistor on RF line.
As I said in the post, there is a 1.8V DC on TCU antenna port, and if you short it to ground, the current is only 0.3mA, so I believe it is for antenna attach detection purpose, i.e. there should be a pull down resistor in the Ford factory antenna (shark fin or PIFA), otherwise there is no way for TCU to detect antenna attached or not.