In the engine compartment, you will find a green wiring plug that runs off of the negative side of your ignition coil. You will need to run a wire from this plug, through the firewall, into the blue connector that plugs into the back of the guage cluster. The blue connector is know as "Connector A" in the factory service manuals. There should be 12 slots in the connector - A group of 5 slots on one side, and a group of 7 on the other. There will be a little plastic divider between the group of 5 and 7 slots. Count over to the sixth slot (through the group of five, and then the very next slot is the sixth.), this is where the wire from the ignition coil plug plugs in. That will get your tach to work.
This next step is absolutely critical - you MUST have the proper oil pressure sender to use with the SR5 guages. If you try to use the oil pressure sender that goes with the non-SR5 guages, you can and will fry the oil pressure guage in your new cluster. I hope that you haven't done this already. The proper oil pressure sender is kind of a round dome shaped sender, rather than the smaller little sender that came on the non-SR5 trucks... The oil pressure sender mounts on the engine block down near the oil filter (below and to the left if I recall correctly).
Other than the above mentioned items, everything else should work, assuming the new cluster is good, and that all of the bulbs are good.