How to "siphon" plutonium

Is it possible to get plutonium out of cars?
I can’t remember

You probably do not, but try bashing a reactor to get some slurry.

I’m assuming you’re talking about Plutonium Cells, used by the Minireactor in some cars.
Yes, you can get those back, under certain conditions.
First, you don’t siphon the Plutonium Cells, you unload them. Then, the game only allows you to unload full plutonium cells. As it currently stands, 1 Plutonium Cell is 500 Charges for the Minireactor, giving you at most 20 Cells at a fully charged Reactor. If the reactor has 1499 Charges, you will only get 2 Cells because 1 of them is already used. If the reactor is at 391 or something like that, you’re not able to get a cell out of it.
I don’t recommend bashing a reactor unless you really need that slurry either. Without proper equipment this is a fast way to get your character killed due to radiation.

You can, iirc it’s the d button.

Ahhhh I was wondering what it meant by no fully charged cells to remove.
So basically unloading will only work if plutonium is over 500 in a reactor?

Basically that, yes. Every 500 Charges you get a Cell out of the Minireactor.
I’m not sure, but the Reactor MIGHT keep the charges if you remove it from the Car, so if it still has some charges left, you might be able to take the reactor, put it on your own vehicle and use the charges below 500 to fill up your batteries.
Or let the reactor run on the old car and take its charged batteries afterwards, both works I guess.