2008 F250SD 4X4 left brake light out

My left brake light is out.  It works for running lights or parking lights but the brake and signal light is out.  It stays lit when the signal is engaged and the front signal lights blink very fast.  I checked the voltage on the circuit and it just fluctuates but never pulses with a consistent voltage.  I plugged my trailer into the connector and the trailer lights do the same thing.  Any ideas?  I thought this would be a simple bulb replacement, but NO! I also checked the wires going into the multi-switch on top of the steering column and they seem to look good.  I tilted the column up and down several times while the left signal was blinking and it never made any difference.