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93 SL500, 95 SL320, 96 S320, 98 S500, 2002 E320 4Matic Wagon & A little 91 5.0 FORD Mustang
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I tested my 98, and mine is pulsing, and the 98/99 cars have really NO relays. Many things are transistor driven. My car is no help to you as it is completely a WHOLE new generation how things are done.

My suggestion is behind the engine fuse panel is a bank of relays on these early to mid 90's (one button key fob cars). With a helper turning the ignition switch on and off find one, or find the relay that turns on the ignition system. Add a relay to that switch-leg with a suppressing diode. On the contacts, grab a constant 12v, and their is your 12v switch for your aftermarket radio.

About the radio staying on, the "volume" control is just shorted on. I replaced many of those volume controls. Some of the momentary push buttons on the radio face panel will short closed too. Those can be replaced too. Ahh you say too much work for an old radio, but many of us want the "OG" (Original Look) instead of "Boom Box Look". I have been toying with modifying our old radios. To do Bluetooth audio is no biggie but to do Call Bluetooth has been a little challenging, but I know some vendors offer this service.

Martin
 

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1995 S500 Coupe
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My car (1995) has the stock MB radio. If Pin 7 has no 12V, then the radio does not work. If your stock radio works fine then you should have 12V on Pin 7 and the issue is with your A/M radio. On the other hand - and as you stated - if you force the relay to close, you have 12V on PIN 7 and then your A/M radio works then I doubt that your stock radio works fine without forcing the relay to close. Follow Martin's advice for a go-around with an extra relay.
Are you aware that the stock radio has a hidden switch inside the connector (radio side) that closes once the female connector is attached to the radio? If you do not use that connector e.g. while testing your radio, you must manually engage the switch or the radio does not work either.
 

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‘95 S600 Sedan 98K miles / ‘95 S420 Sedan 107K miles (sold) / ‘95 E320 Sedan (sold) -- 157K miles
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Discussion Starter · #23 ·
My car (1995) has the stock MB radio. If Pin 7 has no 12V, then the radio does not work. If your stock radio works fine then you should have 12V on Pin 7 and the issue is with your A/M radio. On the other hand - and as you stated - if you force the relay to close, you have 12V on PIN 7 and then your A/M radio works then I doubt that your stock radio works fine without forcing the relay to close. Follow Martin's advice for a go-around with an extra relay.
Are you aware that the stock radio has a hidden switch inside the connector (radio side) that closes once the female connector is attached to the radio? If you do not use that connector e.g. while testing your radio, you must manually engage the switch or the radio does not work either.
I just went back and hooked up the stock radio for the first time in a while to test it. The radio works fine, and powers up upon pushing the power button. The radio comes on even when there is no key in the ignition. Regardless of the stock radio being turn on or off, and either with the key in ACC or ON there is still no voltage at pin #7 on the radio harness.

Throwing money at parts is typically my last approach, but thinking at this stage I will just buy a used N10 to try swapping them out and see if there is any change.

I did test that forward relay on the circuit board for voltage (the same one I could manually manipulate to get voltage at pin #7) and got the following results, sorry the picture is crude. First pic is for reference of the points I measured, then corresponding pic for voltage.

2685070
2685071
 

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1995 S500 Coupe
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I apologize for not having experience with the N10 internals hence my questions: Did you measure each pin to ground? Are there only four relay pins? Please measure which relay pins are connected to Pin 7 of N10.
Which model is your stock radio? I assume Becker 1692?
 

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‘95 S600 Sedan 98K miles / ‘95 S420 Sedan 107K miles (sold) / ‘95 E320 Sedan (sold) -- 157K miles
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Discussion Starter · #25 ·
Just providing an update/resolution on this.

I found a used N10 relay and swapped it out with my existing relay, and just like that I have 12V at the radio harness pin #7 in ACC and ON key position!

The wipers which also previously did not work in the intermittent setting, now also work in the intermittent setting!

In my excitement I didn’t even think to try my heated seats which also had only been working intermittently. Fuse 22 in the engine bay fuse box which feeds the radio also has in parenthesis “heated seats”. I’m wondering if the same relay within N10 that should have been closing the circuit to give pin #7 12V in the radio also serves the heated seats (the intermittent issue with the heated seats is that they just don’t get power at all). I’ll post an update when get out for a drive and can try them with the car on so I don’t drain the battery warming my ass :)
 
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