77 300D Radio Wiring - PeachParts Mercedes-Benz Forum

realmikeyp · #1 07-18-2021, 01:00 PM

I’m trying to install a new radio (Pioneer MVH-S30) in my 1977 300D however the wires are different from every other diagram I found. There is no black wire for 12v ignition so I’m not sure if the other wires are different as well. If anyone could let me know what wire does what it would be very helpful

123boy · #2 07-18-2021, 03:48 PM

Pretty tall order for someone to know, off the top of their head, "what wire does what" in your particular vehicle. Not uncommon for wire colors to not match what is shown in any given diagram. The benefit of the diagram is to show where wires originate and terminate, if they're fused, power/ground, etc. I'd say get a wiring diagram for your car, grab a multi-meter and start wringing out the conductors associated with the radio. Keep track as you go by making a list/diagram and/or marking wires as you discover what they're used for. It shouldn't be too big of a challenge to find the "hot wire with ignition on" for your radio connector. Then again......as far as old cars and electric wiring is concerned, all bets are off. Anything is possible depending on previous owners, modifications, etc. Not what you wanted to hear, I know, but wiring can be a challenge (even when brand new).

jay_bob · #3 07-18-2021, 09:43 PM

Normal convention on the 123 chassis:
red wire is constant battery (may have a stripe on it, this means it's fused)
black wire is switched accessory (again with a stripe on it if it's past a fuse)
brown wire is ground

You'll probably just want to run new wires to each speaker location since the factory wires are pretty wimpy and they go through a fader which is completely incompatible with your amplifier.

The older Beckers did not have any digital electronics so they only had the switched hot wire. Newer 123s had a 2 pin connector under the radio if you got the 'upgraded' digital radio but not sure if they had that in 77. This had a constant hot and an illumination wire.

If it was me I would get a copy of the wiring diagram online for the car and get the voltage sources from the back of the fuse box. (Disconnect the battery first!) and then you can drop the fuse box into the driver foot well to work on it. You will see there are ready sources of switched and unswitched and illumination power there.

123boy · #4 07-19-2021, 08:36 AM

^^^^^ This is great info! "Normal convention on the 123 chassis:
red wire is constant battery (may have a stripe on it, this means it's fused)
black wire is switched accessory (again with a stripe on it if it's past a fuse)
brown wire is ground"

Thanks for posting. I'm still kinda new to this 123 "scene" and your post sheds a lot of light....for me, anyway. Knew about the brown ground, but that was about it as far as wire color constants were concerned in this car. That's the great thing about this forum. I learn something every single time I'm on it!

Good stuff!

ah-kay · #5 07-19-2021, 11:17 AM

I am not sure about your radio but most radios need these wires.

1) constant 12v.
2) switched 12v from ignition.
3) ground. Mbz is brown color.
4) antenna trigger.
5) front, rear, center or subwoofer speaker wires.

You can ohm them out easily or install new wires. Good luck.

resago2000 · #6 07-19-2021, 06:53 PM

you can remove the fader and the wires from the rear speakers should be long enough to pull up to the dash.
I filled the hole where my fader was with a dual usb port too.