Can I add lumbar padding to existing seat?

As well you should be: the airbag connector under the seat is part of the firing circuit for an explosive device. The precautions to take before, during, and after servicing are not complicated, however, and they are all clearly explained in the Repair Manual, though I can’t take the time or liability for summarizing them all here. I’d gladly post the relevant sections for you to read, but if I were to do this, I expect I’d just get PriusChat a copyright takedown notice and myself banned.

If I were you, I’d pay $15 for a two-day subscription to techinfo.toyota.com. You can’t download the complete Repair Manual as a PDF file, but you can view, print, or save as many sections of it as you like, with not only the airbag and other precautions, but also useful advice, like the technique to avoid wrinkles when reattaching the seat covers. (They also have the complete electrical wiring diagrams, which can be saved as PDF files; I found those alone to be worth the cost.) There aren’t any videos for the Prius, but there are lots of illustrations, most created from 3D models of the car and its parts.

In fairness to other posters, it’s true that supplemental restraint systems have built-in features for safe servicing, such as connectors with shorting bars, and many people have worked on them without any attention to the manufacturer’s recommendations. For me, compared to the cost of dealing with an inadvertent airbag deployment (or worse, personal injuries, either during servicing or because the system doesn’t work when needed), the money and time spent to get and follow the correct procedures are trivial.

On their European repair information site (toyota-tech.eu), Toyota does publish accessory installation instructions for leather seats and seat heaters for free viewing. There are some differences between European and U.S. cars and accessories, but those documents might give you a few ideas about how the seats are put together. (Be aware that the European accessory manuals use a pictogram style that is much different from the Repair Manual, which is written in proper English.)

 

It might be too thick - I used a flannel in a previous car - probably only 3 thicknesses, maybe 4. It often doesn't take much to make it comfy. Don't forget, you may need to adjust either the steering wheel (or seat position) to compensate.

 

Need to moves the back straighter up. Otherwise you sit way back.

Posted via the PriusChat mobile app.

 

It took me quite a while to find a comfy position, a little adjustment, further back, forward, wheel closer, backrest, adjust Lumbar - etc.

Then put it in for service, and they'd moved the seat - GROAN!!! back to fiddling again.