I suggest you ohm test just the white wire (359) from the cluster to the temp. sender, that should read 0, since there should be no resistance.
More resistance in the circuit, the colder the gauge reads, so if now it is reading hot would lead to believe the white wire has a break in it and is touching the engine. This test will help confirm that. With the cluster plug unplugged and sender disconnected, connect the ohm meter to white wire at the sender, touch the other probe of the ohm meter to a clean spot some place on the engine, if you get any reading the white wire is bad.
Over the years the white could have rubbed against something and worn off the plastic coating and is now touch something/ground.
When you say - They tested 145 ohms- I am unclear as to how you had the ohm meter connected. Was it ohm meter connected to sender connection and other ohm probe connected to engine? If this is how you had it connected you are now ohm testing the cluster and I have no guess what that is.
More resistance in the circuit, the colder the gauge reads, so if now it is reading hot would lead to believe the white wire has a break in it and is touching the engine. This test will help confirm that. With the cluster plug unplugged and sender disconnected, connect the ohm meter to white wire at the sender, touch the other probe of the ohm meter to a clean spot some place on the engine, if you get any reading the white wire is bad.
Over the years the white could have rubbed against something and worn off the plastic coating and is now touch something/ground.
When you say - They tested 145 ohms- I am unclear as to how you had the ohm meter connected. Was it ohm meter connected to sender connection and other ohm probe connected to engine? If this is how you had it connected you are now ohm testing the cluster and I have no guess what that is.