i have three amps grounded to a back seat bolt with no problem check out partsexpress.com they have a firewall bushing made by streetwires that locks as tightened and creates a waterproof seal

 

I have wires through both firewall holes, plenty of room ive got three sender pairs and a vacum line and some other wires on drivers side, and stereo wires all through the passenger side, and YES ground is bolted down in the back. hope it helps!