I've been having a problem that is been pretty hard to nail down.

My tachometer shows about double of what the actual engine speed is. It has done this before, but I was able to fix it by changing the ignition module.

I changed the module, coil and capacitor and I still have the same problem.

I had the car for about 6 months before this happened. There is a tachometer filter, but it has never been hooked up.

Could this be the problem? How would I wire it in?

The tachometer filter is sitting there on the firewall, with the wires looped around plugged into itself.