That is what confused me you didn't say which bulb location
As said earlier I'd check all sockets for grounding and make darn sure the bulbs actually work correctly.
Once you have done that, if the problem persists it almost has to be either a harness/ connector problem, or a bad TS switch
The connections are:
The connector coming out of the column for the TS switch
The bulkhead connector which feeds the front lamps
The left kickpanel connector which connects the rear harness