Get a meter or a test lamp and troubleshoot - OR - take it to an auto electrician. The headlamp circuit is completely separate to the parking & marker lights.
On these vehicles the headlamps are ground switched, which means the switch is between the headlamp & chassis ground - they are separate fuses for left & right sides, so the fact that your high beams work says the fuses are good - that leaves the bulbs, the wring and the column switch as possible problem areas.
Start with the bulbs - probe the wiring behind the headlamps - there will be three wires - if you're using a trouble light, it should light at full brightness on one wire (the power feed, either white/red or white/blue) and partial brightness on the other two (the switch connection, red & red/white) if you don't get light on all three wired you either have a bad bulb or a bad connection at the socket.
If the bulbs check out, go under the dash where the wiring harness from the column switch comes out of the shroud and find the red & red/white wires and repeat - you should see the same pattern as above - if not there's a wiring problem between where you are and the headlights.
If the bulbs & the wiring checkout - you're left with the column switch.
