This song takes place in a strip club, and is about anal sex, hence "Back Door." Like "Pictures Of Lily," it was an effort to push the limits of what could be inserted lyrically in a song that receives airplay. It did surprisingly well, charting at #61 in the US and getting substantial airplay on rock radio.
Perfect Strangers featured the Mark II lineup of Ian Gillan, Ritchie Blackmore, Roger Glover, Ian Paice and Jon Lord. It was the first Deep Purple album with this lineup since their 1973 effort Who Do We Think We Are.
Suggestion credit: Mark - Londonderry, NH, for above 2