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If tire pressure is set to the correct specification and the light stays on after driving or pressing the reset button then you may have a failed sensor or a module related issue.
There is a sensor in each wheel that may have failed, but the timing is definitely suspect. It could be a failure of a sensor itself or it's internal battery.
Since this happened directly after replacing the radio then it may definitely be related. The tire pressure monitoring module is mounted in the dash directly under the radio, it's possible that the module was unplugged and wasn't plugged back in, it may have somehow been damaged or it's wiring could have been damaged in the process of replacing the radio.
Without disassembling the dash or using a scan tool to see what fault codes have set you've really covered everything you can. I'm assuming that you don't have a scan tool, if disassembling the dash to access the tire pressure monitoring module isn't something you're up for then I would recommend returning to the shop that did the work and see if they can go back in and make sure the module is plugged in and wiring isn't damaged.
I was looking through some other lexus forums, and came across something that I had not seen yet. Under the glove compartment, if you get down under it and look up there is a button that that can be set to 1 or 2, and it's used to indicate which set of tires you're using, this is something useful to owners that need summer tires and winter tires. The guys that were working on my stereo had definitely changed it by accident, and the system could not get a reading on the non-existent set of tires, hence the alert on my dash. When I found that switch, I was so relieved, it all makes perfect sense. Thanks for your work on my issue, and I'm sure that this info. is useful to you as well.