I just upgraded from z140 to 7100 so I can use Google maps and not waste $1200 for pioneer to not update maps after a year...but the z140 had the option to use the rear speakers as a sub and I can't seem to find that on the 7100.

Does anyone have a suggestion on how to achieve this (without buying an amp and installing a sub)? The audio was sounding pretty good with the z140 with the set up I had, and now it just sounds pitiful.

If the wiring doesn't say to do It I would not suggest it. The last radio I knew that did this was the Appradio 1. I have not seen that wiring option in the nex line nor do I see the setting to use the rear input in that fashion. I would not suggest it.

I had to look this up... 

This Pioneer feature was called "Direct Sub Drive" and allowed a pair of 4 ohm 'subwoofers' (rear speakers) to be run off the rear speaker outputs of the head unit in bridged mode (2 ohm load), for more power. 

It's not the best way to get bass, but it seem(ed) like a pretty good idea. Too bad it's no longer offered, but maybe that's a sign that it wasn't a commonly used feature or maybe it resulted in too many repairs or amp failures (my speculation). 

The best way to use your current rear speakers as subwoofers would be to run a small external amp to them with the low pass filter on and tuned to the best frequency for your car.