Loose or intermittent ground doesn’t blow fuses. That’s a dead short.
live short on the + side (to ground) blows fuses.
To the OP, fortunately this is pretty easy to diagnose and trace. You know where the offending wire starts and where it ends.
Almost certainly that aftermarket power lead is shorting out. Only difficulty may be running a new wire.
if you’re lucky it’s not affixed hard anywhere and maybe you can just use the old wire to pull a new one thru.
since you removed the head unit and it still blows, you astutely eliminated some sort of internal short in the radio itself.