Joined: Aug 2009
Location: Colorado Springs
Previously, I had an intake manifold gasket go bad. It caused coolant to leak, which resulted in low coolant in the reservoir tank. There is a little float in there that measures the coolant level and when it's low, it trips the light. When my coolant was low, the float stuck at the bottom, and despite filling up the coolant level after fixing the gasket, the light stayed on. I got a long screwdriver and poked it at the float to free it. Then the light went off.
OP if the light looks like this (see below) it's low coolant. Believe it or not, there are people who know about Mustangs on this forum.