The aftermarket speakers in your truck at the moment are probably higher quality than the infiinity speakers in any event. The head unit will have low-level outputs, as the front amps were on the speakers..... I don't recall where the rear amp was, it's possible its still there....

 

I just wired my tweeters in parallel with the front speakers… works fine.

The tweeters are wired from the amp on the back of the stock front speaker, so the wiring mod is easy.

If you search for my old posts there’s a description of how I wired mine.

[edit] and here it is: https://dodgeforum.com/forum/2nd-gen-ram-tech/337914-upgrading-stock-speakers-2.html#post2878048