It was very popular about 10 years ago. The process involves heating the headlight housing and separating the clear plastic lens from the mirrored portion. Then trimming the housing until there is enough room for the projector assembly and gluing the whole thing back together. In this case usually JB Weld putty. There are lots of vids on youtube. A long time ago there was a lot of crap being sold on the market, like cheap ignitors and ballasts (just like there is a ton of LED crap now), but now most of the bugs have been worked out. Cheap Chinese shit has moved on to something more profitable (again, LED) and you can actually get kits that are quality and reasonably priced. About $150-400 bucks. The BEST LED lights on the market struggle to perform as well as an entry level HID projector.
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The other bonus is nearly unlimited customizability. The reflector was painted satin black with soft flat iron accents to match my bumpers and I chose to have DRL's that mimic the Porsche Panamera (4 white led's surrounding the main light) The DRL's turn amber to double as a turn signal (switchback). I'm not a halo kinda guy so I painted over that portion with red paint to accent other bits of my jeep.
Its all a bit of work, but I have something unique that works extremely well