Stereo is quiet, even at max volume

[Magnus001]

Hey guys, new here, so I don't know if this is common, or not. My friend has a 2011 Sierra, with a stock system (not Bose) and he bought a Kenwood deck, PAC module, and everything. I got it all wired up, and plugged it in to make sure it worked, and it did. Yay! So we proceeded to get it in the dash, and buttoned up, and got two bolts in, to secure the deck. Then turn it on again, just to make sure nothing came unplugged, and it still works, before putting all the facia back on, but now it's super quiet, even if the volume it turned to max. Pulled it back out, checked all connections, pins, and all that. It "seems" to be fine. Please help. Thanks.