ILS back course mode switch

Doesnt seem to be a backcourse mode switch on the in house 737,I see one on the backup asi,but cant seem get it to work?,am I missing something?am flying procedure turns to ILS backcourse approaches, which is fine with the 747 & 172 & Carravan,how do I get the 737 to lock on to a backcourse ILS approach

Sorry if this is old stuff,but am a returnee

Thanks for your help

bobg