The pre-outs are for an eventual amplifier/subwoofer that you would add. So they are pretty much useless unless you buy an amplifier or/and a woofer.
You can try to wire without the negatives. You ought to have sound, since the speakers are already grounded to the chassis (or straight to the bat's negative, I don't know for sure) instead of the head unit.
Another option is to run cables to the speakers to get negatives back in front for a more conventional setup. Some take the occasion to upgrade their speakers to their favourite brand and change cables entirely.