How to use "Put * the Backseat" properly?

The sentence goes like this: I have a few items on my plans, item A is the least important one, so I will put it "at" the backseat, or put it "onto" the backseat?

What preposition should I use in the expression "put _________ the backseat"?

The sentence goes like this:

I have a few items on my plans, item A is the least important one, so I will put it _________ the backseat.

Should I use "at" or "onto" here?