Upallnight asked the things I would check first.

The little wire you describe by the ignition switch sounds like the antenna to receive the code from the key (key has mechanical function AND a wireless function AND the remote entry function - all different) maybe if that is broken it is not recognizing the key - even the new key.

Definitely check the starter relay as well. Did you open the engine electronics box during the work? If so, check for broken pins and missed connectors.

DIS/INPA software may be a little better for troubleshooting something like this if you can get access to it. A little PITA to set up initially.

Post more as you go.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles