How to fix the radar

First thing first: Scan it for codes with an all system scanner - the ADAS system uses C codes, not readily revealed P codes -- my experience with this on an Optima usually points to the mirrors being bad (or one of the bumper sensors, but the code will vary based on which one it is and the location). The code Hyundai ECUs give to find it is a "slave calibration" issue if the mirrors or sensors in the bumper are bad and need to be recalibrated. You can certainly try and clear the codes, but 99% of the time it comes back.

Get the codes and we can do more. That said it’s likely a bad mirror (or bumper sensor) as it is a somewhat common failure on these Hyundai cars with power mirrors and ADAS. I would replace the mirror as an assembly as repairing them is almost impossible with the ADAS mirrors (except the mirror plate, I've saved those from bum mirror assemblies as they can be swapped without complicated disassembly and Hyundai/Kia doesn't sell it, so it will save you if you break the mirror glass). Sometimes a reset and calibration helps but don't count on it. If it lacks ADAS (think BSD and AEB), we need codes to work with as it may not be an ADAS issue, but if the car also has ADAS the cruise control is likely on the same CANBUS network.

As a last resort, check for any damage to the sensors or alignment issues with any on the bumpers or front grille - the Optima I had the issue on hides them under the bumper and mirrors, and the Sonata is basically the Hyundai version of the Optima. I would also be careful about used mirrors -- the electrical ones use a gearbox which is made of plastic and ONE GEAR likes to strip -- and you either pray the next one is good OR you go through a grueling rebuild process. There's YouTube videos on it, but it's not fun... I'll leave it there, if I would rather pay a shop and paint a new mirror, it's bad. If you are careful and pry gently, you might be able to replace the rear shell without painting it but don't count on it - prepare to paint the replacement.

BE AWARE YOU WILL NEED TO PAINT THE MIRROR - don't pay a shop for this. Get some Duplicolor PerfectMatch based on your car's color code, a self etching primer that isn't 2 part and see if you can find a 2K clear that doesn't require breaking an epoxy block inside the can so you can keep the can for a while. This isn't going to be as close of a match as a 2K clear, but it has much better pot life: https://www.eastwood.com/eastwood-diamon... and this type of primer: https://www.eastwood.com/dupli-color-per...

Make sure you use a adhesion promoter on the plastic before adding the primer and paint - plastics need this without exception: https://www.eastwood.com/dupli-color-adh...

Hyundai/Kia ECUs are absolutely nitpicky here in my experience - you need to get the mirror calibrated to the car. Unless you have a bi scanner which supports ADAS pairing (read: $$$$), you will have to find a body shop to calibrate and pair the mirror to the car. Rumor has it OEM Hyundai mirrors auto calibrate, but 3rd party ones do not - those require manual pairing.