I just purchased a 1975 450 SL and am having a problem with the radio.
The seller told me that the radio worked fine, until he had it repaired for
a blinker light problem. The radio turns on and lights up, and the antenna
goes up when I turn it on and goes down when I turn it off, but there is no
sound.
I looked under the driver's side dash and discovered a little box with a red
light and a switch, and a couple of other disconnected wires, but I haven't
got a clue as to what they are there for, or what to do with them.

Can anyone enlighten me?[8|

 

I have a 79 450SLC and have never seen anything under the dash with a red light? Perhaps a alarm system? They have blinking lights wer activited. Did you check the connections to the speakers? If they are connected and you have a OHM meter. connect it to the speakers and see if there is a power them, also check and see if there is a amp somewhere in the car. My 450 had an aftermarket radio and the previous owner added rear subwoofers on the hat shelf. An amp was mounted in the trunk up against the wall that is between the trunk and gas tank. Not the best spot to do this. I had a problem wit the rear speakers going on and off. Tracked the problem to the brown ground wire to the amp. Also, for reasons unknown to me, The local installer grounded the speakers to the car body, a no, no according to the informtion on the amp.

If the speakers are connected, look to see if there is a ground wire for the radio. The Sony in my car has a ground for the radio also. In this case the installer used a yellow wire. But usually the ground connections is clearly marked. This ground you are looking for is for the speakers. The Sony has two grounds one for the radio and one for the speakers up front. Seeing your getting power to everything else, it has to be a ground or a loose or disconnected wire.

Whoever installed the radio in my car was not very good. I cleaned up the mess and used speaker wires instead of ordinary electrical wires the installer used.

 

Been out of town. Thank you so much for your advice! I will let you know if it works!