As a test, do a temp swap of lamp assemblies from side to side. If the problem moves also, you have your answer.
^^^^This. You can also get three wires and make jumpers and connect them directly from the harness and a chassis ground right to the bulb. If it's still dim on that side, put a volt meter on the circuit and see what you're getting compared to the opposite side. If it's not that housing it is likely at the bulkhead the steering column connector or the blinker switch.
