All the Manual transmissions after '85 were 5-speed boxes? I had wondered about this because the 4speed OD unit is not mentioned 1n the 1990 owner's manual. However, the trans looks to have been right there from day one, there's no hacked wiring, all factory connectors well integrated into the harnes, and the whole thing looks to be original. I figured maybe a special ordered car or the factory using up old inventory. When I was looking at other cars to buy, the later ones were all 5-speed, except this one.
If this is the case and someone did a really nice job swapping in a manual transmission, that would explain the funky operation of the light (amber up arrow, btw) it's on when the overdrive is off, off when the overdive is on. If this is in fact what I have, then there may be no 4th gear switch connection. I'll have to have a closer look under the car.

thanks