hey,
i want to take my bmw business rds cassette head unit (philips manufactured), but i also have a BMW Amplifier in the boot/trunk (some sort of premium sound system?)

So do i just use a normal bmw-iso wiring adaptor or do my audio signal need to come from the RCAs on my new head unit?

thanks

A normal BMW-to-ISO adapter will do just fine. You only need the RCA outputs if you intend to change the amplifier in the trunk too.