I wonder if one could build a metal cap, shape and color matched to the sharkfin, that goes over the antenna and couples to the roof of the car. Would that be enough to make a small Faraday cage over the antenna? Would leakage though the mounting hole still be enough to let a signal through?

I've tried stuff like that before on other things and haven't been very successful. Those damn EM waves really like to find their way through.

I don't think you really need to get that sophisticated. If you unplug the antenna and send too much current down that antenna line, whatever's on the other side is not going to like it.

You could maybe just stuff the inside of the sharkfin full of tinfoil.

If it is similar to eCall in VAG ECUs, then there might be internal antenna inside the ECU itself.

What the fuck

This would really suck.