I have a rear turn signal working but not the front. Obviously the flasher is working. The bulb is good and there are no burned or corroded wires or contacts. And I've traced the dead wire right back to the bundle at the steering ccolumn.

Must I assume the switch has separate contacts for front and rear circuits? And that one contact wouldn't carry two lights?

Quite correct, the fault's going to be in your turn signal switch. See this lengthy thread for repair instructions. If that fails, see this thread for full replacement/retrofit details and parts sourcing, etc.

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