I am not sure that this will help you, but i printed this off awhile back b/c i am planning on getting an aftermarket bumper, and like you i want to use the factory
lights. this shows a great way to mount them. it is for a shrockworks bumper, but the reason i kept it was b/c i liked the way he made a bracket for the factory lights.
if there is no bracket to mount to, what you could do is either get one welded on the back side, or drill two holes in the front of the bumper, and mount a piece of salt treated wood, like a 2x2. hit it with spray paint and bam you have something to mount to. that would work great. just bolt the wood down.