2003 4.3 mpi tach and oil pressure

I apologize in advance as I am new to this forum and could not locate my exact concerns/suggestions using the “search”. Serial number on the engine is OM665701. Engine is in a 2003 Bennington tri-toon. Gauges in dash appear to be Bennington (or their choice) as these gauges have no markings of “Merc”. My problem is the tach not working and oil pressure gauge problem did not occur at the same time. 6-7 months apart. (A) Tach never works. Not even sporadically. (B) Oil pressure gauge works random. Either zero pressure or maxed-out. (C) I removed both gauges from the dash, cleaned and inspected all connections.(D) I am having a difficult time (internet) finding the correct oil pressure sending unit. (E) How do I locate the tach feed wire/trigger wire where it attaches to the engine/distributor/etc? I looked and did not see anything oblivious. Merc service manual says I am looking for a gray-colored wire. Thanks for any and all help. LPT

re" ....(B) Oil pressure gauge works random. Either zero pressure or maxed-out... "

Almost always indicative of a flaky ground... the bad news is that the bad ground can be internal to the gauge and not repairable.

RE correct oil sending unit... If you REALLY want to buy another oil sender, plug you engine serial number into this forum's sponsor's parts locator for mercruiser engines...

Tach wire... best bet to find that color wire is under the dash after it exits the prefab merc engine wiring cable. Possible bad connection in that cable's connector where it connects to the engine in a largish multipin connector.