Depending on the aftermarket install, you might just be able to push the HU out. Unless the PO/installer put the DIN cage in the unit is probably just a friction fit. It's unlikely the aftermarket hooks line up with the OE holes to properly engage.

Wire tracing isn't really that big a deal. Commit early to removing one seat, the console (or the wood trim cover at least) and the rear seat. It sounds like a lot but it's really only 8-10 screws/bolts/nuts. The carpet comes up easy. The thin metal panel in the trunk is also only 4x screws.

A working motor is a rare JY find, but I would think a cable would be easy. Maybe reasonable practice to look for the routing, too.

I'd recommend vstech for a motor or parts. I think there are three-wire types and five-wire types.