3 speakers 2 channels

1998 Mazda Protege, Pioneer Premier 50x4 with 3 preouts, hooked up to Pioneer 220W 6x9" in the rear, stock 25w Pioneer in the front.

What I'm wondering is, can I hook up the positive of my 50w RMS Optimus (yes, I know it's garbage) to the right channel rear positive and the negative to the left channel rear negative (or vice versa)? Would this damage my head unit in any way? There are no amps involved.

I'd like some feedback before I risk this. I've already done it with the stock headunit and stock speakers but I just want to be extra certain with this new equipment.

If you want to add another speaker, use an amp.  Otherwise you'll put too low an impedence load on the amp.  It may work for a little while, but it will give you problems in the near future.  Then you'll have to disconnect that speaker, and buy a new deck (and an amp if you still want to add your other speaker).

Okay, thanks for the feedback. I'll pick up a garbage amp for my garbage subwoofer lol

Need something to take some of the stress off my other speakers, I won't need to push them as much to get the depth