The manifold heater
http://i163.photobucket.com/albums/t...00180WinCE.jpg is controlled by the manifold heater relay
http://i163.photobucket.com/albums/t...01421Small.jpg. The relay is powered by the OP switch
http://i163.photobucket.com/albums/t...itchMedium.jpg which on a 258 is located just north of the starter. The OP switch also controls the echoke. There is an orange wire running from the relay
http://i163.photobucket.com/albums/t...01422Small.jpg that connects to the red wire that powers the manifold heater. Hope this helps.