Realise this may well be sorted by now. If it is, what was it?
You can check the vss but most people won't have the equipment to do it, ie a microcontroller with a pulse in function and some minimal programming would do rightly. You could also use the microcontroller to prove out your speedo and converter using a PWM pin.
It's good practice to check the resistance of the wiring too as any water inside the insulation can cause black wire corrosion, which means the wiring more than likely has to be replaced.
You can give all your connections a clean with a glass fibre pencil too, best thing for cleaning electrical connections.
Also of it's crimped style connections make sure they are tight on the male to female and the crimp on the back of the connectors.