Any E46 seats will work. My E53 came with the optional Sport Seats (without lumbar, which I desperately wanted). So, I initially installed Heated, Power M3 seats with lumbar & power adjustable side bolsters. Everything was plug n' play but I ultimately ended-up going with the comfort seats. So, any E53 or E46 will definite work. After all, the X3 is fundamentally just an E46 anyway.

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-Dusty
2007 X3, Montego Blue
1998 Z3 Roadster, Montreal Blue
1996 Z3 Roadster, Jet Black
2016 Miata LE, Soul Red
1981 DeLorean, Stainless Steel
1979 Ferrari 308 GTS, Ocean Blue