Hello,
I recently bought a 1987 740gle wagon, and it had the stock radio. I bought a new radio and wanted to install it. I've installed many radios in cars over the years and expected no issues. Come to find out, as I removed the old stereo, there is a weird plug that didn't fit anything including the adapter. I cut the end of the plug off and now have enough wires to install the new radio, but I have two sets of 6 wires and each set arethe exact same colors (red, white, black, yellow, green, and blue). I also have the grounding wire and another yellow wire. I've searched the Internet and this forum, but have found nothing on this. If you know anything, I would be very appreciative of your advice/knowledge.
Thank you.

 

sounds like you got the wrong adapter.

I am afraid I only have wiring diagrams for 240s and 91+ 740/940's, not for early 740s, so I'm not going to be able to give you the wiring codes.

on both of the 91 and 92 cars I've wired stereos onto, the factory door speakers were connected to an amplifier over near the steering column, which I disconnected and bypassed so the modern stereo amp could run the speakers directly. the factory amp was connected to the factory deck with a DIN connector (round thing, looks like a PC PS/2 keyboard connector). the rectangular connector into my factory stereos had power, antenna, dashboard lights, and the dashboard speakers.

this is the wiring I used for a 1992 740/940...
https://docs.google.com/document/d/1...8io/edit?pli=1

note I used speakers with external crossovers in front, put the tweeters on the dashboard positions and the woofers in the front doors, but conventional coax speakers in the rear doors. I'm not sure I recommend this speaker placement, if I do it again, I'll put the tweeters right next to the woofers, as it is, the bass and treble are a little 'disconnected' and it sounds too bright.

 

The earlier 7's were wired the same as Pierce describes. I've done my 84 760T as well as my 90 740. In both cases, there was an amplifier. On the 84, I wired it up to run through the existing amp when I hit the amp/boost button on the receiver. On the 90, I just wired around it. Modern receivers have more power without an external amp than the early stock setups. So...the existing amp is extraneous, IMO. Seems I had some issues as well with connectors, but it's been long enough my memory is sketchy. I do know I ended up wiring it by hand and using a handful of bullet connectors.

 

the 'good' local car stereo store had the adapter harness for the 740/940 with amplifiers... harness has two connectors, one that plugs in to the factory amp harness, and the other that plugs into the connector that went into the factory deck. wires on these harnesses were color coded exactly the same as modern aftermarket stereos, so made the job easy.

the one I linked with color codes, someone before me had hacked in a stereo by whacking off the factory connectors :-/

and yeah, even a $100 alpine/kenwood/pioneer/jvc will have better amps built in than that 1980s external amp. digital 'class D' switching amplifiers didn't exist in the 80s/90s, so 50Wx4 would have required a massive linear amp. (yes, I know, thats 50W "music power", these modern decks are more like 17-20W RMS per channel into 4 ohms, but thats still PLENTY for any sane system).

 

Translated as any grown up's system. I love seeing $500 cars come in to the shop on tow truck. Toasted alternator and an amp cable going to the amp suitable for an arc welder. Routed poorly, when the insulation rubs through, it can be quite spectacular! July 4th in September! Kids will spend $2-3K on a system and mount it in a total beater.

 

my daughter gave me a ride in "her" 240 yesterday, I'd forgotten how good the speakers were I'd put in that, hah! low end recent pioneer or kenwood (I forget which) deck, feeding JL Audio oversized 6.5" high end coaxials (seperate crossover) in the rear deck, and 4.5" JL's in the factory door mounts. she was playing some bass heavy synth based moody hipster stuff (wasn't EDM, wasn't really rock, kinda reminded me of Enigma but faster and with a beat), and dang it sounded pretty good. daughter (17) was laughing that the sound in her friend's newish jetta was pure crap by comparision, all whumpy with no mid and treble thats too bright, so this sort of music sounded like garbage. of course, she was using her ipod jacked into the deck.... CD's are so last century.

 

I ended up finding the switched 12v+ lead and the ground, so I hooked those up and then taking some red and black wire and routing the two front speakers with that to the back of the stereo. It works, but I have no constant 12v+ to keep memory...I'm still looking into this issue. So as right now I can listen to music through, just can't save any presets, thanks for the advice. I will look for some sort of amp that I have overlooked.

 

in a pinch, over by the steering column, above the driver side kickpanel (remove the kick panel AND the knee bolster pad plus metal brace for easier access), you should find a black rectangular cover about 2x3" that has a whole pile of red wires going into it. this is the main 'positive terminal', and is directly connected to the battery. pop the cover off, and you can attach your lead there. I would use an inline fuse as close to this terminal as practical so that if your radio wiring ever shorts, you'll just blow that fuse, and not burn up the whole wiring harness.