I have a 1985 GT notchback. I secured a complete 1987 fastback with the tail light harness.
When I plugged it in the cooling fan started.

I have figured out that the 1985 notchback and the 1987 fastback are not plug and play for the tail lights.
I have found a few posts that talk about new wiring but they are not all the same.

Does anyone have complete information regarding the wiring requirements to use a 1987 fastback wiring harness in a 1985 GT?

Thanks in advance

some differences between 85 GT & 87 GT wiring are Location of backup light switch and the High Center brake light or lack of it. A plug & play harness may not be an option however the project is feasible if you can read wiring schematics from both vehicles and improvise. A soldering iron and good clean solder are your friends.

Spoon

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Even after repining C500 light section to fit 85 car... brake lights won't work.
You must cut a wire from pedal switch to turn switch & run wire all the way back to brake light wire.

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The Ogre's Fiero Cave

When I swapped 88 GT rear body parts onto my 85 SE, I decided to keep the 85 harness- I do not like yellow REAR turn-indicators, so I used only the red section of the GT taillights- had to do a little filing to make everything fit- they don't quite snap in....I also used a couple of LED bulbs in the inner spots so when you hit the turn-indicator the LED comes on (and turns off) quicker than the outer incandescent bulbs....looks sort of sequential...

Yes, not compatible.

First you need to repin the tail light harness to match the 85 C500 plug (with the exception of J9 as this is for the brake lights). You then will need to remove the wire from the brake light switch on the pedal and rewire it to J9 on the C500 plug.

Now there may be an easy way to reuse the brake light wire from the pedal switch, but I don't have access to that wiring diagram right now, but here are the C500 connectors for the cars:

85 C500 pinout:


86 C500 pinout:


This is easy enough to do as I have done something similar to this before (I will have to see about relinking the pictures): https://www.fiero.nl/forum/...070315-1-061640.html

quote Originally posted by Mickey_Moose: Yes, not compatible. You then will need to remove the wire from the brake light switch on the pedal and rewire it to J9 on the C500 plug.

No. Cut 1 White Wire from pedal switch to turn switch. Then splice new wire to brake side.

Can use a spare "hole" in 85 C500 body side to repin the brake lights. Avoid assigned holes in charts above even if empty for your car.
But Hard to find new pins for C500. If have "doner" body side can use old pin w/ wire as use as a pigtail & splice new wire to that.

All other mod's are done to 87 Light section to fit 85 body side plug.

post 2 add some notes:

1. 86 etc WHT wire, Circuit # 17 @ G7 & H8 do not have = pins on 84 or 85 C500 because is to "program" the car for which type taillight set.
Notchies have a jumper there, GT don't, on light side of C500. Is same as Cutting wht wire & add new wire above.

2. Putting Fastback on 84/85 may confuse others because have no High Brake & may cause odd problems w/ vehicle Inspectors etc.

quote Originally posted by theogre: No. Cut 1 White Wire from pedal switch to turn switch. Then splice new wire to brake side.

Cut - remove doesn't matter, any event you need to run a wire from the brake pedal to the back. If you cut, you just splice the wire together, if remove, you add the pin to the new wire and insert it into the connector.

As I said, I have done this, when I modified my notch back tail lights (done this on an 85 and a 86) - granted I didn't use the fastback wiring harness.

86 is a bit different procedure as was stated.

86 lights (brake, reverse and flashers lit):


85 lights (note: 87 coupe fascias installed):
flashers:

Brake:


The 86 later also received the fastback body and tail light - but the wiring was already modified so I didn't have to change it other than adding another socket for the extra turn bulbs).

[This message has been edited by Mickey_Moose (edited 12-21-2023).]

Here are the 85 wiring diagrams:




Basically:

- Remove center console. Best to remove all covers and the plastic skeleton to allow easy access to this area.
- Find connector C210 - lower right hand side of steering column.
- Remove the pin from the connector (or cut the wire as Ogre said) from the side opposite side that is connected to the brake switch - be sure to install heat shrink over the used end. You will use the end connected to the plug.
- Run a new wire from this point under the console (use the same gauge/size of wire and to keep it consistent with the wiring diagram colors, use white wire), through the firewall, along the harness on the right side of the engine bay (passenger side) to the C500 plug.
- Install this wire in the C500 plug on the car side on PIN J9
- Remove the wire from the tail light harness C500/F0 plug (trunk fan) and install at C500/D1 (or if there is a wire at C500/D1, splice them together) - I don't think there is a wire there, but without seeing a 86 fastback tail light harness I am not sure.

I drilled a small hole (big enough for the brake switch wire to fit) in the plastic bulkhead wire plug on the passenger side and taped/tie wrap this wire along the harness to the C500 plug. Seal the hole with silicon.

Done.

You will need tools to remove the pins from the C500 connector, but Princess Auto has these - or just cut and splice the wires. Just make sure you use water proof heat shrink if you cut and splice wires.

NOTE: C500/F0 means, C500 plug, pin F0 - use the diagrams I posted above to figure out which is which - the plugs are labelled, but can be hard to read. The C500 connector is the big one near the battery that the harness plugs into.

quote Originally posted by Mickey_Moose: 86 lights (brake, reverse and flashers lit):

Is that a 1996-1998 Pontiac Grand Am dashboard I see in there? It looks like it perfectly fits from what I can see...