@jrichker do you have a write up on this, or at least the turn signals?
Mikestang63's post
Tilt wheel tilt 89? into 91lx what is involved? | Mustang Forums at StangNet is actually a link to a post with some very good instructions on this question. I highly recommend that it be used and followed.
This is the second time I have answered the same question for him...
I previously told him to put the 90 switches & wiring on the 87 column. Either he didn't take my advice or it didn't fit; I don't know what he did.
Because these are two different designs, there are some things that may not mate up correctly, like the horn and cruse control. The steering wheel is obviously different and that is were I would expect the problems to arise.
Multifunction turn signal switch:
Before you think about replacing the multifunction switch with one from the junkyard, here are some things to be aware of...
The problem is more common in GT models because they had fog lights on the same power wiring as the headlights. Ford undersized the wire and that caused problems.
A word of caution about multifunction switches is in order here. The multifunction switch (high/low beam, wiper, turn signals) are different for different years. 87-98 will work in any 87-89 car. The 90-93 switches only work in 90-93 cars. You can't put an early model switch in a late model car, nor can you put a late model switch in an early car.
Supposedly you can move the pins around to make the switches work in model years that are different from the car the switch came out of. I cannot verify that and haven’t tried it.
Other possible problem sources for the turn signal & headlight malfunction are the ignition switch, multifunction switch and the plastic shell that holds the turn signal wiring connector pins.
The following diagram is for 87-89 model cars.
Turn signal switch wiring:
The following diagram is for 90-93model cars.
Turn signal switch wiring: