I am in the process of a slow interior restoration on my E53 and the seats are next. LIke yours, my drivers seat is not very comfortable any longer. New leather with anthracite Uberkaro inserts for both fronts are on the horizon, but the lack of comfort really needs to be addressed during the re-covering effort.

I was hoping to buy new foam/upholstery like you, but tried ordering at the usual places and kept being told it was no longer in production. I assume that RealOEM just hasn't caught up? I'd love to replace mine, but as it is will probably have the upholstery shop try to work some magic with foam repair.

The other option is of course to swap in a newer seat that is either in better shape, or for which the seat cushion is still made. I considered a sport seat swap in for a bit, and the guy I buy fabric from for my X5 told me that "the (e53) X5 actually shared seats with the E46 (and also the E60, E63) but not the E39 oddly enough. E60 M5 seats can be had for pretty cheap and look at home in the E53. They are usually in good shape too, they don't seem to wear as bad as E46 or E53 seats." I can't speak to the ease, or lack thereof, of getting the electrical bits in the "new" seats to work with the E53 modules. There are quite a few good seat swap threads on this and other forums though. At any rate, once I started adding up the cost of used seats, new foam, fabric/leather and work, it made more sense to just re-hab and recover my comfort seats.

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Current
Original owner 2002 E53 X5 4.4i to 4.6i swap
2025 G06 X5 50e
Former
1972 Audi Fox
1986 Saab 900S
1996 BMW Z3
1998 BMW E36 M3 Sedan
2004 BMW E46 M3
2006 Audi A3 Quatro
1993 Mopar 318 Jeep Grand Cherokee
2015 V6 Jeep Grand Cherokee