So, I searched around on the forum, and couldn't find much information on converting tail lights from a 90-92 700 to a -89 700.

I was at the junkyard earlier today and found a nice set of 1990 760 tail lights. I decided to take them home and try it out myself.

As previously mentioned, you'll need to hacksaw/cut out the middle bars in your 700's tail light mount (they are flimsy columns of sheet metal on the rear body, you'll see what I mean once you're in there).

Here's the tricky part: Newer taillights ground back to the car's wiring, older tailights ground direct to body. You'll need to take the newer harness from your newer tailights and ground it directly to body, both sides. Ground wire is black and thicker gauge than the other wires.

Here's another tricky part: Parking lamps have two positive 12 volt wires, one runs to the tip of one parking lamp, the other runs to the base of the other parking lamp. This is counter-intuitive and doesn't make sense, but you'll need to trace all the pins of your old tailights, and match the wiring/polarity to the newer tailights. Some positive wires reach the base of their bulb, while other positive wires hit the tip of their bulb. It's not the same with each bulb. With careful inspection, you'll figure out which of the two identical parking lamp wires goes where when you swap harnesses. For the separate white wire with a clip that goes to a small lamp on the old tailights, this one goes directly into the harness on your newer tailight. You have to get this one right for everything to work out (match bulb polarity between old/new lights). I cannot stress this enough, study the polarity of your old tail lamp several times, and match the polarity on your new tail lamp harness (Base of bulb vs. tip).

Rear fog lights are also tricky, you'll get two wires for the driver side, but only one for the passenger side. There are also two wires for reverse lights, which join up on the driver side.

In short, the funky polarity reversals at the tail lights must be what make the bulb failure sensor work properly. It looks like I wired everything up right, as I don't have a bulb failure indicator, and I've confirmed everything works. What a nice solution to the cracked tailights on my '89 744T! You should not have any spare wires leftover, every wire has a matching place between the old and new harness (remember, newer style tailights ground directly to car body, just like the old ones did).