flashing oil light and trim guage not working

[gradywhit]

I have a 2000 yamaha 250hp saltwater series. last summer boat was running well. out ot the blue the trim bars did not show the correct position of the outdrive (the outdrive still would go up and down) at the same time the 3 oil bars had the first bar lit, the second bar flashing and the third bar out. the boat ran fine all summer with this happening ( oil in both tanks). I replaced the sending unit in the oil tank on the engine.and on the boat tank. i replaced the trim switch on the motor. the tach is 4 years old. my thought is that I have a bad wire , but i have no idea how to troubleshoot that. Any help and advise is appreciated.

[boats.net]

Need to test trim sender underneath transom bracket and wiring from sender to tack. Need to test wiring from oil tank sensors to tach.

[gradywhit]

do you know which color wires go where. and the best plan of attack to do this
thanks