If the wires under insulation are not "clean copper", then yeah, solder won't "stick".I know, I’ve already tried solder and heat shrink twice including solder seal connectors and neither worked as I believe the wires are too old and dirty for solder to stick as they just came apart no matter what I did.
You need to:
- check rear left speed signal, that is the one I believe responsible for speed on the cluster
- redo all this mess by soldering and using heat shrinks (or pay someone to do it)
- if the wires are bad, cut/strip them until you get to a nice copper metal. Then replace the bad part of every wire. Make damn sure, you don't mix them.