Finally, some forward progress on the Mustang last night. I managed to get one of the harnesses installed. It's not perfect by any means, but it's in and a lot safer than that single lap belt.
I also got some time to wire brush the passenger side door trim where it was all "rusty". Not sure if it was rust or 40 years of built up arm sweat (gross!) but it came off with a wire brush and wire wheel on the drill. There's a before and after shot below. All I have to do now is get some time on it to get the paint on.
Also got under the rear to check that leaky area in front of the axle. The good news is, it doesn't look like fuel. It is a red oily substance. Looks like transmission fluid to me, but I can't work out what it's doing way back there.