First step is to check for voltage when they aren't working. If you get voltage going into the gauge but nothing is comming up on the gauge then either the gauges are toast or maybe a bad ground. Also, check the connectors for corrosion.
If you aren't geting voltage at the gauges when they are not working then start looking at the sending unit to see if you get voltage out there. Could be a dead short in the circuit or again possible corrosion.
The speedo runs off of a cable that bolts into the side of the trans. This is actually not a wiring issue, its a mechanical issue. You may need a new speedo cable or speedo gear. It may also be the gauge.
The sending unit for the fuel gauge is in the tank I think...