07 Stereo Issues

First off, hello. And hopefully I posted this in the right section.

Been having issues with this 07 (v6) stereo replacement for days now. First off the previous owner cut all the speaker wires. So we had to run those straight into the adapter for the new unit. Which is this little thing: https://www.crutchfield.com/p_070XDVD251/Dual-XDVD251BT.html

Simple and cheap is what I thought with the idea to upgrade later down the road when the car itself gets some upgrades. The stereo finally got thrown in and wired up so it is working. But when we tried to shut the car off the AUX power would kick on for a split second (cause a pop noise) and shut down. But shortly after that the car would then try and turn over? I honestly can't even begin to understand how the hell that happened. The other two plugs (I believe the subs control and the steering wheel controls maybe?) are in their factory state (the guy didn't cut those) and the one I have an adapter for (subs I believe) that plugs into the new stereo. So the only one not in use is the (steering wheel controls?) and it sits there doing nothing.

Any help at all on this would be really helpful. It has been a 3+ day project just to put a radio in between myself and a few friends. Thanks in advance.
 
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I should also mention that this only happens with the stereo on. If you have it plugged in, turn it on, and then turn it off before you turn the car off, this doesn't happen and the car shuts down perfectly normally. And we followed a wiring diagram the best we could as well.