I haven't ever owned a car of your period but you are looking to trace the White/Yellow lead from the SW connection of the coil and you should find it according to the diagram below.

http://www.gomog.com/allmorgan/wiringdiagram1969to1973.jpg


2009 4/4 Sport Henrietta
1999 Indigo Blue +8
2009 4/4 Sport Green prev
1993 Con Green +8 prev