Electrical nightmare

So I hooked up my license plate light a couple weeks ago on my 1953 mercury 6 volt positive ground system. Went to take the car to grab some lunch on day and everything was good, went in to and ate lunch, came out of the restaurant and my car would not start, turn the key and hit the button and nothing. My buddy jumped it with his truck, but I wasn't thinking and hooked the cables up to the battery not the starter side of the solenoid, anyways got it fired up drove it home fine parked it, turned it off then back on no issues. Went out about an hour later and turn the key and hit the start button and nothing. Recharged the battery and checked the charging system, it was charging but not great so I adjusted the regulator. It started great the rest of that day, the next day went out and nothing again when I hit the start button, checked the battery before and it was at 6.3 volts but after I hit the start button it was at 4 something. Load tested the battery and it passed, to it to the auto part store for another check and it passed. Charged the battery when I got home and everything was working again like it should, drove it yesterday made a stop and started fine after that, then went to a restaurant came out and same issue, no crank or start. Tried jumping it the right way with my jumper box and it just cranked, tried another buddy's truck and same thing. I took off the distributor cap to check everything there and it looked fine so I reinstalled it, tried my jumper box and it fired up again so I drove it home and parked it. Any ideas of what could be the issue? It's just weird it started after I hooked up the license plate light, and I did it the way it's ran on the wiring diagram
 
When you hit the starter switch, do you hear the solenoid click. If not, try a touching a wire from battery ground to the small switch terminal on the solenoid. If that gets it cranking, you have a starter switch problem; most likely the switch itself degrading, but could be a loose connection.

If the solenoid clicks but no crank, check the voltage on the starter side of the solenoid when the starter switch is on. If no voltage, the solenoid is bad. If full battery voltage, you have a bad connection to the starter; check the starter cable connections. If voltage drops below battery voltage, it could be a starter or engine ground problem, or worn out starter brushes or armature.
 
When you hit the starter switch, do you hear the solenoid click. If not, try a touching a wire from battery ground to the small switch terminal on the solenoid. If that gets it cranking, you have a starter switch problem; most likely the switch itself degrading, but could be a loose connection.

If the solenoid clicks but no crank, check the voltage on the starter side of the solenoid when the starter switch is on. If no voltage, the solenoid is bad. If full battery voltage, you have a bad connection to the starter; check the starter cable connections. If voltage drops below battery voltage, it could be a starter or engine ground problem, or worn out starter brushes or armature.

When I hit the start switch absolutely nothing happens, no click no cranks just silence
 
is the Merc automatic or a stick shift? if automatic the neutral safety switch may be bad or need adjustment.
 
Have you checked and cleaned all of the battery, soleniod & starter connections? Lot of good advice in #6.
 
If you turn the key and get nothing, try smacking the solenoid with a screwdriver or wrench. As noted above, it may be failing. Or you can make a jumper wire to put 6volts directly to it. I did both when the solenoid on my '53 Ranch Wagon started going bad, and I had to wait for a replacement. I received a poor quality Chinese (Ford script) solenoid from a vendor that just chattered when energized, so I just put on a NAPA one and I was back in business.
 
If your light had a short in the circuit it would likely either pop the circuit breaker or blow the fuse depending on protection for it. It should have no effect on things unless its on all the time and draining battery current. It's may have been a coincidence.

Jumping a 6-volt with another 6-volt is OK but jumping with 12-volt is not advised.
 
With the engine off, key off, lights off ... remove the negative battery post connection and put an ammeter from it to the battery. It should read under 1 mA.



Object = is an unknown device draining the battery. Ammeter should read zero, dead zero.
 
With the key on by pass the solenoid by using the jump leads from one terminal to the other or from the battery terminal to the starter terminal ,You will get some sparks but this will tell you if the solenoid is faulty or perhaps the dash button .Ted
 
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