The last verse to me seems to describe the way he is so horny & with no outlet for it that he can't get his mind on any other topic. 'Wherever he tries to hide' i.e. mentally in his thoughts - his lustful yearnings will seek him out & get his mind back on emptying his sack. The wicked harmonica riff seems to convey his agitation & frustration at his curent plight.
The last verse to me seems to describe the way he is so horny & with no outlet for it that he can't get his mind on any other topic. 'Wherever he tries to hide' i.e. mentally in his thoughts - his lustful yearnings will seek him out & get his mind back on emptying his sack. The wicked harmonica riff seems to convey his agitation & frustration at his curent plight.