I am looking to replace my car system and speakers. There is a 4 digit code though attached to my head unit as an anti-theft mechanism. Can I still replace the sterio safely?

You're gonna replace the "System" and speakers,Becker's Security will have
no effect on your "New" system.

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'84 300SD sold
124.128

So when I replace the old Casette Unit with a CD Unit, that security system will just be disabled and I will not need to enter the code ever?

You can't just replace the casette unit with a CD unit. There are no speaker wires or antenna lead at the head unit.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Of course I would be pulling all of the old wiring out that is attached to the optional cd player, old built in phone and 2band police scanner. The wiring needs to be updated for that.