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The harness will not be plug and play. 87-89 are interchangeable in your body. The chassis/dash harness plugs will be different.
 

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There is a slight differences in the 87-88 harness and 89 harness.

The secondary harnesses like taillights and doors and fuel pump relay are the same from 87-89 but the two main harnesses, dash and engine have a few small differences in that in how certain wires are routed. One major different of course is 87-88 is speed density and ‘89 is mass air.

since this is the pro-m forum and I don’t know what your intent is, I’ll just say that for the best compatibility all the harnesses should be from the same year.
ProM offers 87-89, 90 and 91-93 for their harness options. Since his car is 93, he needs the 91-93 for plug and play compatibility. I personally would like to know how they offer a harness from 91-93 when there are differences between 91 and 93 harnesses. When it comes to doing a swap, 91 to mid 92 are the same and mid 92 through 93 are the same. I went through this when I was converting my fox to a 5.0. I tried an 89, 90 and 93 harness before finding an early 92 harness that plugged into all of the body connections. 87-89 makes sense cause you can plug an 89 harness into an 87-89 and it will work assuming the user has a maf ecu to go with it.
 

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Does anyone know what a new ProM harness for coil on plug that goes in a 1989 would cost? I don't see the harness listed on their website, assuming it just comes with the kit.
Thanks
I believe the entire harness is different which is why people recommend getting all the options you think you may want at the time of ordering. I dont think its an addon. You essentially would need to rebuild the harness to work with COP or buy a whole new harness from Pro-M.
 
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