I did this with a drill attachment scrubber. You can use any drill, like a plug in one with no batteries and plenty of power. Works great... I can run through a line of grout in seconds.
I have this: https://www.amazon.com/dp/B003TU662Y
Use it with cleaners, but I just used water at some points works amazing.
Not my video but I have the same exact thing. It’s a brush from chemical guys and I’m sure there’s other similar ones that are different brands.
Chemical Guys ACC_201_BRUSH_MD Medium Duty Carpet Brush with Drill Attachment, Yellow https://www.amazon.com/dp/B003TU662Y/ref=cm_sw_r_cp_api_i_chYICbRXJ5SV4
It attaches to a drill the same way a screwdriver bit does. There’s a hex sticking out of it that the drill clamps on to.
I bet a drill with a carpet brush and with some APC would do some good work on the seats.
Make sure to follow-up with reconditioning them after. You can put a coating on all of it too.
Did you power wash or just use a regular hose?
I got both from amazon. They work so well.
Chemical Guys CARPET BRUSH W/DRILL ATTACHMENT - MEDIUM DUTY - YELLOW https://www.amazon.ca/dp/B003TU662Y/ref=cm_sw_r_cp_apip_k3zKwCmEwanvm
ULTIMATE GROUT CLEANER: Best Grout Cleaner for Tile and Grout Cleaning, Acid-Free Safe Deep Cleaner & Stain Remover for Even the Dirtiest Grout, Best https://www.amazon.ca/dp/B000Y232BI/ref=cm_sw_r_cp_apip_EIDdNrQ4jYuhS
Don't hose them, you'll soak the foam and it will take forever to dry, and you won't be able to suck out any of the filth that will soak into the foam.
I just cleaned my 19 year old Toyota Tundra fabric interior - fabric seats and carpets. I bought a brush and fabric cleaner from amazon. I liberally sprayed a section of fabric with the fabric cleaner diluted according to directions, then used the brush attached to a drill to loosen the dirt. Then, while still wet, I used a rug cleaner attachment and went over the same section with a cleaner and the rug soap that came with it (I used a Bissell Pet Pro Carpet cleaner we bought for the dogs).
I think your plan will work, but just don't soak them too much.
This is the one I bought: http://www.amazon.com/gp/product/B003TU662Y