That's very nice of you to thoroughly clean his car but it is more difficult than its worth to remove the seats. You can easily clean underneath if you just move them all the way up and forwards and then up and backwards.
Specifically, in order to remove the seat, you will have to move it all the way forward, unbolt the back bolts, move the seat back, and undo the front bolts. Then unhook the negative battery cable before unplugging the connector. The seats are very heavy so you may struggle getting them out. The steering column has to be all the way up and forwards to get them out. Be careful when taking them out not to scratch the car. Put towels over the inside of the front doors and on the outside of the body behind the doors. Reinstall is reverse. But honestly though, the seats move enough that you can clean under them without removing them. I always do a very meticulous clean and so when I switched out my seats, there was barely anything underneath.
Mine spotless without removing. First push them forward before lifting the up. Take a brush and go underneath the track really well so that you can vacuum all the gunk up. For the side that closest to the middle, I take an old gift card to get any change and debris form it. Vacuum and I'm done.
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