Prius Prime pre-collision system and headlights

That's also listed at McGeorge Toyota as a US spec part. I don't think there is any involvement with a headlight lens and the pre-collision system, which uses a combination of the camera in the windshield and radar sensors behind the Toyota emblem on the front of the car.

 

I'm not sure how that sensor would be effective behind the bumper fascia, particularly a curved part of the fascia that would be in front of that. Check and see if has a part number on it.

 

There may be different firmware in the headlight ECU sub-assembly to support the automatic high beam system, which uses the forward recognition camera found only on cars with the pre-collision system.

If you have a car built for sale in the U.S., it might not be possible: the headlight assemblies seem to be sealed one-piece units. In the U.S. version of parts catalog Figure 81-01, Headlamp, note that no part number is listed for part name codes 81131B, 81132B, or 81131A, only for the complete headlight assembly RH, part name code 81135, part number 81145-47800.

I wouldn’t read too much into that; the parts websites seem to list any part in the Toyota price file, regardless of whether it’s actually orderable or used on vehicles built for the U.S. market.

On the contrary: many radar sets use the same antenna for transmitting and receiving. Toyota’s design— described in Kawakubo, A., Tokoro, S., Yamada, Y., Kuroda, K. et al., “Electronically-Scanning Millimeter-Wave RADAR for Forward Objects Detection,” SAE Technical Paper 2004-01-1122, 2004—uses separate, phased-array antennas, but as seen in the internal photos of the millimeter-wave radar sensor assembly on Prius and Prius Prime cars, they are part of the same module, mounted behind the emblem in the front bumper assembly.

 

I cant tell from the pic and i havent disassembled mine, but as i work at another oem and my equipment calibrates this tech, i can promise that little pad has likely nothing to do with it. Its probabaly a moisture vent. The windshield camera sends a signal to the adas module, who then talks to the main ecm cpu, and finally the bcm which controls all the relays. Beyond that equipment, i also had headlight aim and ive taken every model lamp apart we make (5 models) and there's only ever the driver board to control the low and high beam.

 

I was thinking that looked like it might be an air bag crash sensor. Than I looked at the part diagram.
The diagram didn't rule it out completely, also didn't confirm it with any real certainty either, so check #4 in the sensor section at the bottom of the parts list page.
Genuine OEM Air Bag Components Parts for 2019 Toyota Prius Prime Plus - Olathe Toyota Parts Center