You need to go over to MyMopar if you have not and download a 71 shop manual. Plymouth, Dodge, does not matter
Now there is a MISTAKE for several years concerning this circuit in the shop manuals. The T and L pin of the bulkhead connector is mis-drawn. BUT IT WOULD have been correct when original. Just look at what pin the green / red goes to on the engine side, and that is where it should come out of on the interior, unless someone has messed with the connector. IT MAY BE that the bulkhead connector itself is where you are losing connection. You should be able to trace the green/red from the relay to the horns. One way is to simply jumper battery power right to the green/ red at the relay. The horns should operate or a test light at the horn end should light
Judging from the crimp connectors in your photo..........the wiring HAS been fiddled with?
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Download the 71 Plymouth chassis manual:
http://www.mymopar.com/downloads/servicemanuals/1971_Plymouth_Chassis_Service_Manual.zip
The interior side is shown page 8-160 (381) and the engine bay side is 8-151 (372)