The big red msd power wire is coming off the big lug on the starter. The big black msd ground wire is is grounded to the body underneath the battery tray.
You should probably check that ground connection closely, maybe even try connecting to the battery. It could be just a dirty, oxidized connection that combined with the voltage droop from cranking is keeping the MSD from working as it used to. Areas around the battery are prone to oxidation.
