First, bring the discussion down to facts. It's not that you don't want to deal with another task or you feel you won't handle it but you know it and you can support it with hard data.
Until you start discussing facts you may be facing the issue that PM is just adding things to your backpack and wait until it's clearly seen you have too much. There can be many reasons of such attitude, starting with a lack of trust and finishing with lack of will to make difficult decisions on task priorities. Either way with no data you won't solve this.
Second, visualize that. If you tell me that you're overloaded and you have dozen tasks in backlog it doesn't say as much as your task board would say.
I don't say it has to be task board or Kanban board. It can be whatever visualization tool which works fine and shows data you want to discuss.
Third, use meaningful historical data. Backup your point with experience from the past. Find similar situations and show how overloading team with tasks ended up in late delivery or random choice of tasks or chaos in priorities. Show how effectively you could work at times when the team wasn't overloaded with tasks etc.
One thing I want to stress is data should be meaningful, meaning it should be about this very team, in similar and comparable situation etc. It shouldn't be some made up information which just show what you want it to show. You have to believe in value of the data.
Fourth, show consequences of not changing PM's attitude. If nothing else works I retreat to a discussion like this: if you don't set meaningful priorities or give us enough information how we should choose priorities we would end up choosing tasks mostly randomly, as we aren't equipped well enough to make reasonable choices. Is that what you want? In other words if everything is equally important, everything is equally unimportant as well.
Sometimes, even such argument doesn't change the approach and you basically have to deal with the situation by yourself, trying to make right decisions while facing insufficient information. You can consider good old escalation to someone higher in a pecking order - either your boss or PM's superior. Depending on a person it can help, although basically the discussion would look the same. The only difference would be person taking part in it would hold more power and more authority.