The Buddha does reject the autonomous person conventionally. It is not a question of finding a place where he does so explicitly, as it is about realising the disjuncture between the kind of conventional person posited by the Buddha, and the kinds of autonomy westerners tend to mean when they say autonomy. Sure there is still a conventional person of some kind, but the kind of conventional person given (by the Buddha) is - necessarily - a process and relational conventional person. i.e. there is no moral autonomy of the kind favoured by western theologians or philosophers, grounded in a concept of soul or rationality or will or transcendental ego. There is perhaps something akin to what is favoured by the British empiricists - a dispositional theory of agency where there is some kind of autonomy found in choice making....but this is still a very social conception of agency.
First, when I say autonomous person, I am referring to a person, the most irreducible nominal basis of which is a unique and independent mind stream, with a unique and specific karma, as well as unique and specific causes and conditions. Invoking karana hetu [each and everything is a cause for all other things apart from itself] etc. is too broad and is an overapplication of the principle.
Autonomy is essential to the definition of "person" [pudgala]. A convention is understood on the basis its definition. Buddha deconstructed persons via the devices of skandhas, āyatanas and dhātus, etc. Nevertheless, karma ripens only on an autonomous person. So it is difficult to argue that Buddha denied autonomous persons conventionally.
The question of dependent origination probably lies at the heart of this conversation; this is where the two truths become important. I think that the Dharma leads us into an apprehension of the dependently originated nature of things, not away from dependent origination per se. But I'm fairly sure you think otherwise - and this is probably the reason for our disagreement.
The function of the Dharma is to end samsaric dependent origination i.e. --> affliction --> action --> suffering, etc. This in turn is based upon afflictive obscurations. Afflictive obscurations in turn are based on knowledge obscurations, and the root of those is the habit of "I am".
This habit of "I am" (unreal as its supposed basis of designation may be) is sufficient for considering ordinary persons autonomous, since it is this very habit that gives them the capacity to act as autonomous agents i.e. acting solely with reference to their own interests.
The process of politics is entirely afflictive and afflicted, as far as I can tell, based on various false senses of identity, "I am", "We are", etc.
Obviously the Buddha does not instruct us to join a political party - but it does not follow from that that the Dharma is distinct from politics. For many reasons - namely that politics is not reducible to party politics and that a contemporary Buddhist cannot read the Buddha's advice in the Nikaya's and apply it as if we are still in ancient India (i.e. obviously there was not party politics taking place there).
All politics is reducible to parties with different sorts of self-oriented goals, including the politics of deep ecology (which has a self-oriented goal i.e. the preservation of the earth's ecosphere for all beings). But even saving the planet is not a Dharma goal. The goal of Dharma is concerned solely with the liberation of persons from samsara. If we extend this to Mahāyāna, still, all Mahāyāna schools are concerned with the liberation of persons from samsara.
The political process at all levels may be used to beautify samsara or control samsara, but politics is ultimately samsaric, that is the point of differentiating Dharma and politics.