Help!I have a 97 Ford Explorer and the problem is my left turn signal will not work with lights on I have checked the few things I can think of (I know very little about vehicles) what could be wrong?

I would start by looking at the old harness.

As Bmark,said check harness and bulb socket it self,may be burnt out.

Remove the bulb from the socket. Insert the two multimeter pins (one black one red) into the positive and negative slots. Set the meter to twelve volts or lower. Turn on the headlights then engage the turn signal. See if the needle or digital readout cycles from zero volts to twelve. If it does, the problem is with the connection between the bulb and socket. If not, begin checking backward.