I am not sure of the wring on these cars, but on some other cars I have seen people move the headlight power to the accessory side. But this is done when its know the accessory wiring can handle the load. Its the quickest and least intrusive way since it just requires, usually, moving a wire behind the fuse panel back. No need for adding a relay and more wire.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car