Tinfoil.
They use the resonating frequency of tinfoil to generate a response to their egg code locator.
Say you’ve got a baked potato, covered in tinfoil, a great Faraday shield, right? Well, yes, until someone cuts a hole in the top. They’ve just turned their potato into an intergalactic homing beacon for a Grey egg. Within milliseconds, a transporter transponder has painted the potato, and the Greys’ egg, about the size of a grain of sand, and with a protective coating, has been inserted into the potato. A person eats it, and the cyst passes through the stomach and small intestines, and lodges in the appendix.
The pupa gestates for three months, until it is about ready to molt, after which it passes through the remainder of the host’s digestive system, and is sent into the sewer system, eventually getting to the “water treatment plant” (actually a Grey nursery!).
Those pupae, after their following molts, eventually become Warrior caste Greys. The Eldar caste Greys need a more exotic upbringing. Selected humans are targeted by the Orbital Mind Control Lasers, and given a particular psychosis. They begin to believe that they must protect themselves by covering their head in — you guessed it — tinfoil. Some believe they’re preventing the mind controls by doing so. If they only knew!
Similar to the potato case, with a partial tinfoil shell wrapped about their cranium, the person’s head then becomes the target site. The Eldar cysts are then transported directly into the victims’ brains. After they have eaten their fill, the pupae begin to migrate, passing down the spinal column, and finally boring through a large intestine wall, and leaving via a similar route as the Warriors.
Of course, in both cases, there are occasional mishaps which require medical attention. Cysts getting stuck, medicinal interactions, etc. But, then, they can usually guide their human lackeys to coax things back on track.