Touching the supply to the radio to the radio ground effectively reversed the polarity and semiconductors hate that as it destroys many of them in microseconds.
You might be lucky and find the only damage is some burned printed circuit tracks - but it could be much, much worse...
I would recommend you purchase a new harness. Connecting a wire to the individual pins and then insuring that none of the leads touch another is difficult at beat. Replacement power/speaker harnesses are only $7 to 10. A simple google search showed several vendors.
the esn is only active if the previous owner activated it. the user would of used a CD to be a key cd and that is how they are unlocked. if the cd is lost or damaged, Eclipse can give a code to unlock it. However the original owner needs to have registered the deck with eclipse and have some kind of ID like mother maiden name. there is also a second registration that can be done. which an eclipse dealer can do, it is for used cars with eclipse radios, and things like that.
They can be real strict sometimes about giving out a code, and the authorized dealer has to be the one calling, and somtime that means a lot of time on hold.