HU outputs are bridged amplifiers. There is no "ground" wire. Both neg and positive outputs are active. If you touch the neg speaker output to ground, you could instantly smoke the drivers on that side.
Running the speaker return to ground is not a good thing to do. Dunno if it'll damage anything, that depends on the HU amp design. You will definitely get only half the output voltage (1/4 the power) assuming nothing smokes.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's