Wow, sorry for the late responses, lots of work, seems like I have more work now I have retired, hehehe.
Anywho, 16 volts is stressing not only the battery but the any electrical device, including manual switches. Remember E-I-R, so the more voltage the more heat on any resistance in a circuit.
So, first you need to do a voltage drop test, voltmeter negative to negative, then with the vehicle running, and accessories off, check the voltage at the output of the alternator, then check it at the battery, if you are off more than .8 V, you have a wiring issue.
Next, with the vehicle off, set your DVOM to ohms, clean a spot on the alternator, and check from the alternator body to the negative terminal on the battery, your resistance should be almost 0. If you find high resistance in the ground, you need to clean the negative at the battery and the engine block. By the way, you should have a ground direct to the block, some have a ground to a bracket, not the best, better direct to the block.
Also if you have these, get rid of them
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And use these INSTEAD
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Also if you don't have one of these, get one like it, clamp style for amps, it will help in finding draws, and is fine for basically everything automotive
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Anyway hope is helps
Blue