the 71-73 sockets on the grill suffer from corrosion. It is possible to take them apart and rebuild them but it isn't easy. The socket is impossible to find. I had to take a set from the 73 mach 1 and re purpose them to work on my 72.
just FYI the 73 versions besides having 3 connections because they double as turn signals, also sit deeper then the 71-72s, the sockets look the same until you put them next to each other and you find out the 73 socket is about 10mm longer which moves the light bulb back into the housing more then the 72 and 71.
When i converted the 73s i found to 72s i figured out how to pop the connectors out of the sockets and clean them up i had to pull them out anyway to wire them into my engine harness correctly.
they kind of look like RC car battery terminals but they are thinner, so you can use a micro flat head screw driver to bend the side ears in on the socket connectors and slide the connector out of the sockets.
now you could also use a Rust remover like rust off or rust away it uses Acid the acid eats the rust only and leaves the good metal. leave it in the acid for a few hours then spray the whole thing with electrical cleaner and use some dielectric grease to keep the water from corroding it again.