Replacing software with giraffe would turn the argument into an analogy, which usually are imperfect (ridiculously so for giraffe/tomato). Replacing software with part and head-unit with car however is valid, because the software *is* a part of the car - it is not an analogy.
If you accept that the parts of a car must be serviceable by arbitrary 3rd parties, then the software must be. Otherwise, you must argue only /some/ parts should, and/or some not. In which case, the argument you use for why software should not be must not apply to those other parts (otherwise, your argument is non-sensical).
That software is not mechanical is such an argument. But why does that matter? If by mechanical, you mean the solid stuff, well the oil isn't that either. That's seems a strangely arbitrary argument.
As for complexity, the car in its entirety may well be just as complex as the software (excluding the software). The car in its entirety certainly must be least as complex as the software, so the extension of your argument to the car would mean that /no/ component of it should be serviceable or modifiable.