Have a look at the plugs on the back of the bulbs first,unplug them and check for any corrosion.
Next get your handbook and find out which fuses and relays they are on and unplug and inspect them for loose connections or corrosion.
All you can do if those are all ok is to check the various multi pin plugs in the circuit and the headlight and dipswitch plugs and switches,try putting a small amount of pressure sideways on them and see if the lights come back on.
Have you looked at the bulbs ?.
Are they the same when actually switched on via the headlight switch as when they are on the drl.
Vaughan