Updating the radio in a 1970 Dodge Dart with standard dash
You could always splice into you speaker wiring and install an equalizer with aux input. Something similar to this:
https://jet.com/product/detail/2773...d=403-184373&gclid=CL3l1Ke7p8oCFQEcaQod9NQDzA
I've never installed one of these, but just an idea of a cheap way to do it.
Print