Top fender mounted turn signal issues

I have never worked on a 74 (year of your Valiant?), but in my 69 Dart, I recall the top "turn indicators" were just wired in parallel to the lower lamps. Indeed, it was a single "hot" wire, with return thru the body. Same deal in my 65 C-body. I did use a 74 Dart engine & light harness in rewiring my 65 Dart. As I recall, it had single "bullet" connectors going to additional turn lamps which were probably the indicators. That connector is probably what you thought was a diode. It should just pull apart.

Your problem is probably either a bad bulb (likely) or a bad ground. In my Newport, the bulb is in a removable socket, which I only realized after tediously removing the whole indicator housing. Don't know about A-bodies, but you could search for photos on ebay. You might replace it with an LED bulb that should last forever, since they aren't easy to access.