Need to extend bulb life on 13vDC, 100 watt headlights
The lights apparently draw about 8A at 13V if they are rated 100W @13V. 

You could drop the voltage slightly by using two or three silicon diodes in series. The total current for two will be about 16A, with a surge of perhaps 150A on turn-on. You will need a fairly hefty diode to withstand that. To take one example, consider two Vishay VS-40EPF06PBF diodes in series.  

They will typically drop about 1V each at 16A if you keep them reasonably cool. 



It looks like they have an excellent chance of withstanding the surge current too (though perhaps not the fault current when a lamp fails). 



If you're going to drop 2V at 16A, that's 32W to get rid of. If you can find a fairly large piece of aluminum to bolt them to, you'll be fine. 

A cheaper and simpler solution is to use a 50W chassis mount resistor. 
A 0.15 ohm resistor such as a 9-1625984-3 from TE is less than $4. At 16A it will drop 2.5V and dissipate 40W. Again, you'll need a fairly large heatsink or it it will die, but the resistor can be allowed to run hotter than the diodes (100°C is okay). 

Personally, I'd probably opt for two resistors (one per lamp) if that was convenient for the wiring, and still use the 50W size. Of course they're going to be double the value each, and the granularity of value choice is a bit better (.25 or 0.3 ohm, so drop will be 2V or 2.4V each). 



Note that if your lamps are halogen type running them at below rated voltage may result in darkening of the bulb because the halogen cycle won't operate properly- but in this case you're mostly trying to get them to rated voltage.