Ok that isn't the amp that's the old radio.

The long cord that came with the radio plugs into the antenna. Nothing plugs into the factory box in the back in that spot. The antenna that used to go into that plugs into the harness cable that is run from the front during the install.

The antenna wire goes from the (radio in the dash) > (the cable seicane provided) > (the other end of the cable seicane provided) > (the factory antenna wire in the hatch area)

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Just BMW's - All cars and trucks was two pages long
1994 318is
1997 528i
2000 323i
2001 X5 3.0 auto
2005 X5 3.0 auto
2011 535xi auto
2013 X5 xdrive35 Turbo
More are at my website

Aftermarket E53 Radio Install