DaveW wrote and illustrated a step-by-step guide for a high-level brake light in his "How To" series. Even it the point of attachment is a bit different, the parts about the electrical connection are probably still highly relevant.

http://www.talkmorgan.com/ubbthreads.php/topics/40612/Trads_High_level_brake_light_w#Post40612

Doug
1985 4/4 4str, Jubilee Blue