Here's what it looks like from the back side.
This is a standard dash on my "work bench", and it has a rallye cluster sitting in it.
The two large plugs you see near the center of the pic are the two main things that need attention.
The gray plug is the cluster light and gauge connections, and needs to have it's pins "broken out" into individual wire runs. You will need to add some orange light wires because the standard harness has one wire in that round connector that powers several bulbs, and the rally cluster uses individual runs. Fortunately, just to the left of the pic, is a three way light splitter (bullet connector) that only has one of the three in use.
The black plug is the wiper connections, and needs to have it's wires extended so they reach the rally wiper switch location. You can see the switch over on the right.
- - - Updated - - -
I just happen to have an extra dash, so I thought I'd mock mine up.
All of this should be real easy to do with the dash still in the car.
In fact, I'm making an effort to not cut any factory wires. I'm sure I'll be successful on that.
I'm just making extensions with male connectors on one end and females on the other.
The gray plug is the most difficult to do, but I found a van at a JY with the same connector, so I can use the pins from that.
I'm taking the black wiper plug off (without cutting the wires), and plan to move it to the end of my extension.
Plan A is to find a donor plug, so the spade connectors can stay clipped in. If not I can live without the plastic housing.