Sorry,but I have not heard of this problem with melting of wires at the
rear engine/firewall. Dont worry though.There are many electrical genuesis
here on the list. Being the weekend, may delay some help for you.  Maybe
search the archives for others that have had this problem. Few have ever
had a problem that cannot be found in these archives.Just click on the word
archives,and enter words that describe your problem.Best to use few,but
specific words , and then weed out the responses.

Any other symptoms? Maybe the starter motor got hot.I have seen this in
other vehicles. The wires will melt, and the starter is at the location you
described.

Roger