Thanks John. As always, your replies are very helpful. The PO had the wiring to the BAT posts reversed between the horizontal and vertical relays (see photo below), but the diagram helped me verify the other connections. I did reconnect everything as per the diagram, as well as switching the relays to eliminate the relay as the problem. The horizontal motor worked fine connected to the suspect relay.
Connected the motor directly to a power source and the red/green combination seems fine, but the yellow/green combination does not. I had taken the motors apart for cleaning, lube, etc. so maybe I created the problem. Tomorrow night's project.
BTW, where did that diagram come from? I have the Electrical Assembly Manual and the Wiring Diagram Manual, but your diagram is easier to follow and see the connections to the relays more clearly.
Thanks again (and thanks to Ray for helping me post photos directly).
1960 Tbird Convertible
Thunderbird Registry #54651