Solution will depend upon:-
1 Wattage of DLR,s
2 Type of bulb (LED, HID, normal filament bulb, etc)
Low wattage normal filament bulbs could be done with a resistor
High wattage fillament bulbs could be done with a voltage regulator
HID might not be possible without modifying the existing balast controler
LED's will depend on how they are powered already.
Peter