Fuel pump relay seems to work, but only getting 6 volts to the fuel pump. Where do I begin to troubleshoot this problem. Thanks.

Trace from where you find the 6 volts back through the tank selector switch and then relay until you find the 12 volts that you need.
Somewhere along the way, you'll find a poor connection, either on the power side or the ground side. Both power and ground circuits need to be clean and free from corrosion to deliver the needed power to run the pump(s).